1.
Crescent
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A crescent shape is a symbol or emblem used to represent the lunar phase in the first quarter, or by extension a symbol representing the Moon itself. It is used as the symbol for the Moon. It was also the emblem of Diana-Artemis, and hence represented virginity, in Roman Catholic Marian veneration, it is associated with the Virgin Mary. From its use as roof finial in Ottoman era mosques, it has become associated with Islam. The crescent symbol is used to represent the Moon, not necessarily in a particular lunar phase. When used to represent a waxing or waning lunar phase, crescent or increscent refers to the waxing first quarter, the crescent symbol was long used as a symbol of the Moon in astrology, and by extension of Silver in alchemy. The astrological use of the symbol is attested in early Greek papyri containing horoscopes, in the 2nd-century Bianchinis planisphere, the personification of the Moon is shown with a crescent attached to her headdress. Its ancient association with Ishtar/Astarte and Diana is preserved in the Moon representing the female principle, as such, it belongs to the class of figures known as lune in planar geometry. The tapering towards the points of intersection of the two arcs are known as the horns of the crescent, the word crescent is derived etymologically from the present participle of the Latin verb crescere to grow, technically denoting the waxing moon. Unicode encodes a crescent at U+263D and a decrescent at U+263E, the Miscellaneous Symbols and Pictographs block provides variants with faces, FIRST QUARTER MOON WITH FACE at U+1F31B and LAST QUARTER MOON WITH FACE at U+1F31C. The crescent shape is used to represent the Moon, and the Moon deity Nanna/Sin from an early time, visible in Akkadian cylinder seals as early as 2300 BC. The crescent was used in the iconography of the Ancient Near East and was used transplanted by the Phoenicians in the 8th century BC as far as Carthage in modern Tunisia. The crescent and star also appears on coins of South Arabia. The combination of star and crescent also arises in the Ancient Near East, representing the Moon and Ishtar and it was inherited both in Sassanian and Hellenistic iconography. In the iconography of the Hellenistic period, the crescent became the symbol of Artemis-Diana, numerous depictions show Artemis-Diana wearing the crescent Moon as part of her headdress. The related symbol of the star and crescent was the emblem of the Mithradates dynasty in the Kingdom of Pontus and was used as the emblem of Byzantium. The crescent remained in use as an emblem in Sassanid Persia, in the Crusades it came to be associated with the Orient and was widely used in Crusader seals and coins. It was used as a charge by the later 13th century
2.
Triton (moon)
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Triton is the largest natural satellite of the planet Neptune. It was discovered on October 10,1846, by English astronomer William Lassell and it is the only large moon in the Solar System with a retrograde orbit, an orbit in the opposite direction to its planets rotation. At 2,700 kilometres in diameter, it is the seventh-largest moon in the Solar System, because of its retrograde orbit and composition similar to Plutos, Triton is thought to have been a dwarf planet captured from the Kuiper belt. Triton has a surface of frozen nitrogen, a mostly water-ice crust, an icy mantle. The core makes up two-thirds of its total mass, Triton has a mean density of 2.061 g/cm3 and is composed of approximately 15–35% water ice. Triton is one of the few moons in the Solar System known to be geologically active, as a consequence, its surface is relatively young with sparse impact craters, and a complex geological history revealed in intricate cryovolcanic and tectonic terrains. Part of its surface has geysers erupting sublimated nitrogen gas, contributing to a nitrogen atmosphere less than 1/70,000 the pressure of Earths atmosphere at sea level. Triton was discovered by British astronomer William Lassell on October 10,1846, a brewer by trade, Lassell began making mirrors for his amateur telescope in 1820. When John Herschel received news of Neptunes discovery, he wrote to Lassell suggesting he search for possible moons, Lassell did so and discovered Triton eight days later. Lassell also claimed to have discovered rings, although Neptune was later confirmed to have rings, they are so faint and dark that it is doubtful that he actually saw them. Triton is named after the Greek sea god Triton, the son of Poseidon, the name was first proposed by Camille Flammarion in his 1880 book Astronomie Populaire, and was officially adopted many decades later. Until the discovery of the second moon Nereid in 1949, Triton was commonly referred to as the satellite of Neptune. Lassell did not name his own discovery, he successfully suggested the name Hyperion, previously chosen by John Herschel. Triton is unique among all large moons in the Solar System for its orbit around its planet. Most of the irregular moons of Jupiter and Saturn also have retrograde orbits. However, these moons are all much more distant from their primaries, and are small in comparison, Tritons orbit is associated with two tilts, the inclination of Neptunes spin to Neptunes orbit, 30°, and the inclination of Tritons orbit to Neptunes spin, 157°. Tritons orbit precesses forward relative to Neptunes spin with a period of about 678 Earth years and that inclination is currently 130°, Tritons orbit is now near its maximum departure from coplanarity with Neptunes. Tritons rotation is locked to be synchronous with its orbit around Neptune
3.
Voyager 2
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Voyager 2 is a space probe launched by NASA on August 20,1977, to study the outer planets. Part of the Voyager program, it was launched 16 days before its twin, Voyager 1, on a trajectory that took longer to reach Jupiter and Saturn but enabled further encounters with Uranus and it is the only spacecraft to have visited either of the ice giants. Voyager 2 is now in its mission to study the outer reaches of the Solar System and has been operating for 39 years,7 months and 17 days. It remains in contact through the Deep Space Network, at a distance of 114 AU from the Sun as of April 5th,2017, Voyager 2 is one of the most distant human-made objects, along with Voyager 1, New Horizons, Pioneer 10 and Pioneer 11. The probe was moving at a velocity of 15.4 km/s relative to the Sun as of December 2014 and is traveling through the heliosheath, upon reaching interstellar space, Voyager 2 is expected to provide the first direct measurements of the density and temperature of the interstellar plasma. The spacecraft would be designed with redundant systems to ensure survival through the entire tour, by 1972 the mission was scaled back and replaced with two Mariner-derived spacecraft, the Mariner Jupiter-Saturn probes. To keep apparent lifetime program costs low, the mission would include only flybys of Jupiter and Saturn, as the program progressed, the name was changed to Voyager. The primary mission of Voyager 1 was to explore Jupiter, Saturn, Voyager 2 was also to explore Jupiter and Saturn, but on a trajectory that would have option of continuing on to Uranus and Neptune, or being redirected to Titan as a backup for Voyager 1. Upon successful completion of Voyager 1s objectives, Voyager 2 would get an extension to send the probe on towards Uranus. Collectively these instruments are part of the Attitude and Articulation Control Subsystem along with redundant units of most instruments and 8 backup thrusters, the spacecraft also included 11 scientific instruments to study celestial objects as it traveled through space. Built with the intent for eventual interstellar travel, Voyager 2 included a large,3.7 m parabolic, when the spacecraft is unable to communicate with Earth, the Digital Tape Recorder can record about 64 kilobytes of data for transmission at another time. The spacecraft was built with 3 Multihundred-Watt radioisotope thermoelectric generators, each RTG includes 24 pressed plutonium oxide spheres and provides enough heat to generate approximately 157 watts of power at launch. Collectively, the RTGs supply the spacecraft with 470 watts at launch, for more details on the Voyager space probes identical instrument packages, see the separate article on the overall Voyager Program. The Voyager 2 probe was launched on August 20,1977, by NASA from Space Launch Complex 41 at Cape Canaveral, Florida, two weeks later, the twin Voyager 1 probe would be launched on September 5,1977. However, Voyager 1 would reach both Jupiter and Saturn sooner, as Voyager 2 had been launched into a longer, more circular trajectory, Voyager 2s closest approach to Jupiter occurred on July 9,1979. It came within 570,000 km of the cloud tops. It discovered a few rings around Jupiter, as well as volcanic activity on the moon Io, the Great Red Spot was revealed as a complex storm moving in a counterclockwise direction. An array of other storms and eddies were found throughout the banded clouds
4.
Ancient Greece
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Ancient Greece was a civilization belonging to a period of Greek history from the Greek Dark Ages of the 12th-9th centuries BC to the end of antiquity. Immediately following this period was the beginning of the Early Middle Ages and this was followed by the period of Classical Greece, an era that began with the Greco-Persian Wars, lasting from the 5th to 4th centuries BC. Due to the conquests by Alexander the Great of Macedonia, Hellenistic civilization flourished from Central Asia to the end of the Mediterranean Sea. Classical Greek culture, especially philosophy, had a influence on ancient Rome. For this reason Classical Greece is generally considered to be the culture which provided the foundation of modern Western culture and is considered the cradle of Western civilization. Classical Antiquity in the Mediterranean region is considered to have begun in the 8th century BC. Classical Antiquity in Greece is preceded by the Greek Dark Ages and this period is succeeded, around the 8th century BC, by the Orientalizing Period during which a strong influence of Syro-Hittite, Jewish, Assyrian, Phoenician and Egyptian cultures becomes apparent. The end of the Dark Ages is also dated to 776 BC. The Archaic period gives way to the Classical period around 500 BC, Ancient Periods Astronomical year numbering Dates are approximate, consult particular article for details The history of Greece during Classical Antiquity may be subdivided into five major periods. The earliest of these is the Archaic period, in which artists made larger free-standing sculptures in stiff, the Archaic period is often taken to end with the overthrow of the last tyrant of Athens and the start of Athenian Democracy in 508 BC. It was followed by the Classical period, characterized by a style which was considered by observers to be exemplary, i. e. classical, as shown in the Parthenon. This period saw the Greco-Persian Wars and the Rise of Macedon, following the Classical period was the Hellenistic period, during which Greek culture and power expanded into the Near and Middle East. This period begins with the death of Alexander and ends with the Roman conquest, Herodotus is widely known as the father of history, his Histories are eponymous of the entire field. Herodotus was succeeded by authors such as Thucydides, Xenophon, Demosthenes, Plato, most of these authors were either Athenian or pro-Athenian, which is why far more is known about the history and politics of Athens than those of many other cities. Their scope is limited by a focus on political, military and diplomatic history, ignoring economic. In the 8th century BC, Greece began to emerge from the Dark Ages which followed the fall of the Mycenaean civilization, literacy had been lost and Mycenaean script forgotten, but the Greeks adopted the Phoenician alphabet, modifying it to create the Greek alphabet. The Lelantine War is the earliest documented war of the ancient Greek period and it was fought between the important poleis of Chalcis and Eretria over the fertile Lelantine plain of Euboea. Both cities seem to have suffered a decline as result of the long war, a mercantile class arose in the first half of the 7th century BC, shown by the introduction of coinage in about 680 BC
5.
Ancient Greek astronomy
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{{Refimprove|date=October 2011 Greek astronomy is written in the Greek language in classical antiquity. Greek astronomy is understood to include the ancient Greek, Hellenistic, Greco-Roman and it is not limited geographically to Greece or to ethnic Greeks, as the Greek language had become the language of scholarship throughout the Hellenistic world following the conquests of Alexander. This phase of Greek astronomy is known as Hellenistic astronomy. During the Hellenistic and Roman periods, much of the Greek and non-Greek astronomers working in the Greek tradition studied at the Musaeum, the development of astronomy by the Greek and Hellenistic astronomers is considered by historians to be a major phase in the history of astronomy. Greek astronomy is characterized from the start by seeking a rational, physical explanation for celestial phenomena, most of the constellations of the northern hemisphere derive from Greek astronomy, as are the names of many stars, asteroids, and planets. It was influenced by Egyptian and especially Babylonian astronomy, in turn, it influenced Indian, Arabic-Islamic, references to identifiable stars and constellations appear in the writings of Homer and Hesiod, the earliest surviving examples of Greek literature. Though neither Homer nor Hesiod set out to write a scientific work, some stars rise and set, others are ever-visible. At certain times of the year, certain stars will rise or set at sunrise or sunset, speculation about the cosmos was common in Pre-Socratic philosophy in the 6th and 5th centuries BC. Anaximander described a cylindrical earth suspended in the center of the cosmos, philolaus the Pythagorean described a cosmos with the stars, planets, Sun, Moon, Earth, and a counter-Earth —ten bodies in all—circling an unseen central fire. Such reports show that Greeks of the 6th and 5th centuries BC were aware of the planets and speculated about the structure of the cosmos. Also, a detailed description about the cosmos, Stars, Sun, Moon and the Earth can be found in the Orphism, which dates back to the end of the 5th century BC. The name planet comes from the Greek term πλανήτης, meaning wanderer, five planets can be seen with the naked eye, Mercury, Venus, Mars, Jupiter, and Saturn, the Greek names being Hermes, Aphrodite, Ares, Zeus and Cronus. Sometimes the luminaries, the Sun and Moon, are added to the list of naked eye planets to make a total of seven, since the planets disappear from time to time when they approach the Sun, careful attention is required to identify all five. Observations of Venus are not straightforward and they eventually came to recognize that both objects were the same planet. Pythagoras is given credit for this realization, in classical Greece, astronomy was a branch of mathematics, astronomers sought to create geometrical models that could imitate the appearances of celestial motions. This tradition began with the Pythagoreans, who placed astronomy among the four mathematical arts, the study of number comprising the four arts was later called the Quadrivium. Although he was not a mathematician, Plato included the quadrivium as the basis for philosophical education in the Republic. He encouraged a younger mathematician, Eudoxus of Cnidus, to develop a system of Greek astronomy, platos main books on cosmology are the Timaeus and the Republic
6.
Sun
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The Sun is the star at the center of the Solar System. It is a perfect sphere of hot plasma, with internal convective motion that generates a magnetic field via a dynamo process. It is by far the most important source of energy for life on Earth. Its diameter is about 109 times that of Earth, and its mass is about 330,000 times that of Earth, accounting for about 99. 86% of the total mass of the Solar System. About three quarters of the Suns mass consists of hydrogen, the rest is mostly helium, with smaller quantities of heavier elements, including oxygen, carbon, neon. The Sun is a G-type main-sequence star based on its spectral class and it formed approximately 4.6 billion years ago from the gravitational collapse of matter within a region of a large molecular cloud. Most of this matter gathered in the center, whereas the rest flattened into a disk that became the Solar System. The central mass became so hot and dense that it eventually initiated nuclear fusion in its core and it is thought that almost all stars form by this process. The Sun is roughly middle-aged, it has not changed dramatically for more than four billion years and it is calculated that the Sun will become sufficiently large enough to engulf the current orbits of Mercury, Venus, and probably Earth. The enormous effect of the Sun on Earth has been recognized since prehistoric times, the synodic rotation of Earth and its orbit around the Sun are the basis of the solar calendar, which is the predominant calendar in use today. The English proper name Sun developed from Old English sunne and may be related to south, all Germanic terms for the Sun stem from Proto-Germanic *sunnōn. The English weekday name Sunday stems from Old English and is ultimately a result of a Germanic interpretation of Latin dies solis, the Latin name for the Sun, Sol, is not common in general English language use, the adjectival form is the related word solar. The term sol is used by planetary astronomers to refer to the duration of a solar day on another planet. A mean Earth solar day is approximately 24 hours, whereas a mean Martian sol is 24 hours,39 minutes, and 35.244 seconds. From at least the 4th Dynasty of Ancient Egypt, the Sun was worshipped as the god Ra, portrayed as a falcon-headed divinity surmounted by the solar disk, and surrounded by a serpent. In the New Empire period, the Sun became identified with the dung beetle, in the form of the Sun disc Aten, the Sun had a brief resurgence during the Amarna Period when it again became the preeminent, if not only, divinity for the Pharaoh Akhenaton. The Sun is viewed as a goddess in Germanic paganism, Sól/Sunna, in ancient Roman culture, Sunday was the day of the Sun god. It was adopted as the Sabbath day by Christians who did not have a Jewish background, the symbol of light was a pagan device adopted by Christians, and perhaps the most important one that did not come from Jewish traditions
7.
Moon
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The Moon is an astronomical body that orbits planet Earth, being Earths only permanent natural satellite. It is the fifth-largest natural satellite in the Solar System, following Jupiters satellite Io, the Moon is second-densest satellite among those whose densities are known. The average distance of the Moon from the Earth is 384,400 km, the Moon is thought to have formed about 4.51 billion years ago, not long after Earth. It is the second-brightest regularly visible celestial object in Earths sky, after the Sun and its surface is actually dark, although compared to the night sky it appears very bright, with a reflectance just slightly higher than that of worn asphalt. Its prominence in the sky and its cycle of phases have made the Moon an important cultural influence since ancient times on language, calendars, art. The Moons gravitational influence produces the ocean tides, body tides, and this matching of apparent visual size will not continue in the far future. The Moons linear distance from Earth is currently increasing at a rate of 3.82 ±0.07 centimetres per year, since the Apollo 17 mission in 1972, the Moon has been visited only by uncrewed spacecraft. The usual English proper name for Earths natural satellite is the Moon, the noun moon is derived from moone, which developed from mone, which is derived from Old English mōna, which ultimately stems from Proto-Germanic *mǣnōn, like all Germanic language cognates. Occasionally, the name Luna is used, in literature, especially science fiction, Luna is used to distinguish it from other moons, while in poetry, the name has been used to denote personification of our moon. The principal modern English adjective pertaining to the Moon is lunar, a less common adjective is selenic, derived from the Ancient Greek Selene, from which is derived the prefix seleno-. Both the Greek Selene and the Roman goddess Diana were alternatively called Cynthia, the names Luna, Cynthia, and Selene are reflected in terminology for lunar orbits in words such as apolune, pericynthion, and selenocentric. The name Diana is connected to dies meaning day, several mechanisms have been proposed for the Moons formation 4.51 billion years ago, and some 60 million years after the origin of the Solar System. These hypotheses also cannot account for the angular momentum of the Earth–Moon system. This hypothesis, although not perfect, perhaps best explains the evidence, eighteen months prior to an October 1984 conference on lunar origins, Bill Hartmann, Roger Phillips, and Jeff Taylor challenged fellow lunar scientists, You have eighteen months. Go back to your Apollo data, go back to computer, do whatever you have to. Dont come to our conference unless you have something to say about the Moons birth, at the 1984 conference at Kona, Hawaii, the giant impact hypothesis emerged as the most popular. Afterward there were only two groups, the giant impact camp and the agnostics. Giant impacts are thought to have been common in the early Solar System, computer simulations of a giant impact have produced results that are consistent with the mass of the lunar core and the present angular momentum of the Earth–Moon system
8.
Natural satellite
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A natural satellite or moon is, in the most common usage, an astronomical body that orbits a planet or minor planet. In the Solar System there are six planetary satellite systems containing 178 known natural satellites, four IAU-listed dwarf planets are also known to have natural satellites, Pluto, Haumea, Makemake, and Eris. As of January 2012, over 200 minor-planet moons have been discovered, the Earth–Moon system is unique in that the ratio of the mass of the Moon to the mass of Earth is much greater than that of any other natural-satellite–planet ratio in the Solar System. At 3,474 km across, Earths Moon is 0.27 times the diameter of Earth, the first known natural satellite was the Moon, but it was considered a planet until Copernicus introduction of heliocentrism in 1543. Until the discovery of the Galilean satellites in 1610, however, galileo chose to refer to his discoveries as Planetæ, but later discoverers chose other terms to distinguish them from the objects they orbited. The first to use of the satellite to describe orbiting bodies was the German astronomer Johannes Kepler in his pamphlet Narratio de Observatis a se quatuor Iouis satellitibus erronibus in 1610. He derived the term from the Latin word satelles, meaning guard, attendant, or companion, the term satellite thus became the normal one for referring to an object orbiting a planet, as it avoided the ambiguity of moon. In 1957, however, the launching of the artificial object Sputnik created a need for new terminology, to further avoid ambiguity, the convention is to capitalize the word Moon when referring to Earths natural satellite, but not when referring to other natural satellites. A few recent authors define moon as a satellite of a planet or minor planet, there is no established lower limit on what is considered a moon. Small asteroid moons, such as Dactyl, have also been called moonlets, the upper limit is also vague. Two orbiting bodies are described as a double body rather than primary. Asteroids such as 90 Antiope are considered double asteroids, but they have not forced a clear definition of what constitutes a moon, some authors consider the Pluto–Charon system to be a double planet. In contrast, irregular satellites are thought to be captured asteroids possibly further fragmented by collisions, most of the major natural satellites of the Solar System have regular orbits, while most of the small natural satellites have irregular orbits. The Moon and possibly Charon are exceptions among large bodies in that they are thought to have originated by the collision of two large proto-planetary objects. The material that would have placed in orbit around the central body is predicted to have reaccreted to form one or more orbiting natural satellites. As opposed to planetary-sized bodies, asteroid moons are thought to form by this process. Triton is another exception, although large and in a close, circular orbit, its motion is retrograde, most regular moons in the Solar System are tidally locked to their respective primaries, meaning that the same side of the natural satellite always faces its planet. The only known exception is Saturns natural satellite Hyperion, which rotates chaotically because of the influence of Titan
9.
Asteroid
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Asteroids are minor planets, especially those of the inner Solar System. The larger ones have also been called planetoids and these terms have historically been applied to any astronomical object orbiting the Sun that did not show the disc of a planet and was not observed to have the characteristics of an active comet. As minor planets in the outer Solar System were discovered and found to have volatile-based surfaces that resemble those of comets, in this article, the term asteroid refers to the minor planets of the inner Solar System including those co-orbital with Jupiter. There are millions of asteroids, many thought to be the remnants of planetesimals. The large majority of known asteroids orbit in the belt between the orbits of Mars and Jupiter, or are co-orbital with Jupiter. However, other orbital families exist with significant populations, including the near-Earth objects, individual asteroids are classified by their characteristic spectra, with the majority falling into three main groups, C-type, M-type, and S-type. These were named after and are identified with carbon-rich, metallic. The size of asteroids varies greatly, some reaching as much as 1000 km across, asteroids are differentiated from comets and meteoroids. In the case of comets, the difference is one of composition, while asteroids are composed of mineral and rock, comets are composed of dust. In addition, asteroids formed closer to the sun, preventing the development of the aforementioned cometary ice, the difference between asteroids and meteoroids is mainly one of size, meteoroids have a diameter of less than one meter, whereas asteroids have a diameter of greater than one meter. Finally, meteoroids can be composed of either cometary or asteroidal materials, only one asteroid,4 Vesta, which has a relatively reflective surface, is normally visible to the naked eye, and this only in very dark skies when it is favorably positioned. Rarely, small asteroids passing close to Earth may be visible to the eye for a short time. As of March 2016, the Minor Planet Center had data on more than 1.3 million objects in the inner and outer Solar System, the United Nations declared June 30 as International Asteroid Day to educate the public about asteroids. The date of International Asteroid Day commemorates the anniversary of the Tunguska asteroid impact over Siberia, the first asteroid to be discovered, Ceres, was found in 1801 by Giuseppe Piazzi, and was originally considered to be a new planet. In the early half of the nineteenth century, the terms asteroid. Asteroid discovery methods have improved over the past two centuries. This task required that hand-drawn sky charts be prepared for all stars in the band down to an agreed-upon limit of faintness. On subsequent nights, the sky would be charted again and any moving object would, hopefully, the expected motion of the missing planet was about 30 seconds of arc per hour, readily discernible by observers
10.
Planet
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The term planet is ancient, with ties to history, astrology, science, mythology, and religion. Several planets in the Solar System can be seen with the naked eye and these were regarded by many early cultures as divine, or as emissaries of deities. As scientific knowledge advanced, human perception of the planets changed, in 2006, the International Astronomical Union officially adopted a resolution defining planets within the Solar System. This definition is controversial because it excludes many objects of mass based on where or what they orbit. The planets were thought by Ptolemy to orbit Earth in deferent, at about the same time, by careful analysis of pre-telescopic observation data collected by Tycho Brahe, Johannes Kepler found the planets orbits were not circular but elliptical. As observational tools improved, astronomers saw that, like Earth, the planets rotated around tilted axes, and some shared such features as ice caps and seasons. Since the dawn of the Space Age, close observation by space probes has found that Earth and the planets share characteristics such as volcanism, hurricanes, tectonics. Planets are generally divided into two types, large low-density giant planets, and smaller rocky terrestrials. Under IAU definitions, there are eight planets in the Solar System, in order of increasing distance from the Sun, they are the four terrestrials, Mercury, Venus, Earth, and Mars, then the four giant planets, Jupiter, Saturn, Uranus, and Neptune. Six of the planets are orbited by one or more natural satellites, several thousands of planets around other stars have been discovered in the Milky Way. e. in the habitable zone. On December 20,2011, the Kepler Space Telescope team reported the discovery of the first Earth-sized extrasolar planets, Kepler-20e and Kepler-20f, orbiting a Sun-like star, Kepler-20. A2012 study, analyzing gravitational microlensing data, estimates an average of at least 1.6 bound planets for every star in the Milky Way, around one in five Sun-like stars is thought to have an Earth-sized planet in its habitable zone. The idea of planets has evolved over its history, from the lights of antiquity to the earthly objects of the scientific age. The concept has expanded to include not only in the Solar System. The ambiguities inherent in defining planets have led to much scientific controversy, the five classical planets, being visible to the naked eye, have been known since ancient times and have had a significant impact on mythology, religious cosmology, and ancient astronomy. In ancient times, astronomers noted how certain lights moved across the sky, as opposed to the fixed stars, ancient Greeks called these lights πλάνητες ἀστέρες or simply πλανῆται, from which todays word planet was derived. In ancient Greece, China, Babylon, and indeed all pre-modern civilizations, it was almost universally believed that Earth was the center of the Universe and that all the planets circled Earth. The first civilization known to have a theory of the planets were the Babylonians
11.
Solar System
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The Solar System is the gravitationally bound system comprising the Sun and the objects that orbit it, either directly or indirectly. Of those objects that orbit the Sun directly, the largest eight are the planets, with the remainder being significantly smaller objects, such as dwarf planets, of the objects that orbit the Sun indirectly, the moons, two are larger than the smallest planet, Mercury. The Solar System formed 4.6 billion years ago from the collapse of a giant interstellar molecular cloud. The vast majority of the mass is in the Sun. The four smaller inner planets, Mercury, Venus, Earth and Mars, are terrestrial planets, being composed of rock. The four outer planets are giant planets, being more massive than the terrestrials. All planets have almost circular orbits that lie within a flat disc called the ecliptic. The Solar System also contains smaller objects, the asteroid belt, which lies between the orbits of Mars and Jupiter, mostly contains objects composed, like the terrestrial planets, of rock and metal. Beyond Neptunes orbit lie the Kuiper belt and scattered disc, which are populations of trans-Neptunian objects composed mostly of ices, within these populations are several dozen to possibly tens of thousands of objects large enough that they have been rounded by their own gravity. Such objects are categorized as dwarf planets, identified dwarf planets include the asteroid Ceres and the trans-Neptunian objects Pluto and Eris. In addition to two regions, various other small-body populations, including comets, centaurs and interplanetary dust clouds. Six of the planets, at least four of the dwarf planets, each of the outer planets is encircled by planetary rings of dust and other small objects. The solar wind, a stream of charged particles flowing outwards from the Sun, the heliopause is the point at which pressure from the solar wind is equal to the opposing pressure of the interstellar medium, it extends out to the edge of the scattered disc. The Oort cloud, which is thought to be the source for long-period comets, the Solar System is located in the Orion Arm,26,000 light-years from the center of the Milky Way. For most of history, humanity did not recognize or understand the concept of the Solar System, the invention of the telescope led to the discovery of further planets and moons. The principal component of the Solar System is the Sun, a G2 main-sequence star that contains 99. 86% of the known mass. The Suns four largest orbiting bodies, the giant planets, account for 99% of the mass, with Jupiter. The remaining objects of the Solar System together comprise less than 0. 002% of the Solar Systems total mass, most large objects in orbit around the Sun lie near the plane of Earths orbit, known as the ecliptic
12.
Neptune
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Neptune is the eighth and farthest known planet from the Sun in the Solar System. In the Solar System, it is the fourth-largest planet by diameter, the planet. Neptune is 17 times the mass of Earth and is more massive than its near-twin Uranus. Neptune orbits the Sun once every 164.8 years at a distance of 30.1 astronomical units. It is named after the Roman god of the sea and has the astronomical symbol ♆, Neptune is not visible to the unaided eye and is the only planet in the Solar System found by mathematical prediction rather than by empirical observation. Unexpected changes in the orbit of Uranus led Alexis Bouvard to deduce that its orbit was subject to perturbation by an unknown planet. Neptune was subsequently observed with a telescope on 23 September 1846 by Johann Galle within a degree of the predicted by Urbain Le Verrier. Its largest moon, Triton, was discovered shortly thereafter, though none of the remaining known 14 moons were located telescopically until the 20th century. The planets distance from Earth gives it a small apparent size. Neptune was visited by Voyager 2, when it flew by the planet on 25 August 1989, the advent of the Hubble Space Telescope and large ground-based telescopes with adaptive optics has recently allowed for additional detailed observations from afar. Neptunes composition can be compared and contrasted with the Solar Systems other giant planets, however, its interior, like that of Uranus, is primarily composed of ices and rock, which is why Uranus and Neptune are normally considered ice giants to emphasise this distinction. Traces of methane in the outermost regions in part account for the blue appearance. In contrast to the hazy, relatively featureless atmosphere of Uranus, Neptunes atmosphere has active, for example, at the time of the Voyager 2 flyby in 1989, the planets southern hemisphere had a Great Dark Spot comparable to the Great Red Spot on Jupiter. These weather patterns are driven by the strongest sustained winds of any planet in the Solar System, because of its great distance from the Sun, Neptunes outer atmosphere is one of the coldest places in the Solar System, with temperatures at its cloud tops approaching 55 K. Temperatures at the centre are approximately 5,400 K. Neptune has a faint and fragmented ring system. On both occasions, Galileo seems to have mistaken Neptune for a star when it appeared close—in conjunction—to Jupiter in the night sky, hence. At his first observation in December 1612, Neptune was almost stationary in the sky because it had just turned retrograde that day and this apparent backward motion is created when Earths orbit takes it past an outer planet. Because Neptune was only beginning its yearly cycle, the motion of the planet was far too slight to be detected with Galileos small telescope
13.
Exoplanet
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An exoplanet or extrasolar planet is a planet that orbits a star other than the Sun. The first scientific detection of an exoplanet was in 1988, HARPS has discovered about a hundred exoplanets while the Kepler space telescope has found more than two thousand. Kepler has also detected a few thousand candidate planets, of which about 11% may be false positives, on average, there is at least one planet per star, with a percentage having multiple planets. About 1 in 5 Sun-like stars have an Earth-sized planet in the habitable zone, the least massive planet known is Draugr, which is about twice the mass of the Moon. There are planets that are so near to their star that they take only a few hours to orbit, some are so far out that it is difficult to tell whether they are gravitationally bound to the star. Almost all of the planets detected so far are within the Milky Way, the nearest exoplanet is Proxima Centauri b, located 4.2 light-years from Earth and orbiting Proxima Centauri, the closest star to the Sun. The discovery of exoplanets has intensified interest in the search for extraterrestrial life, there is special interest in planets that orbit in a stars habitable zone, where it is possible for liquid water, a prerequisite for life on Earth, to exist on the surface. The study of planetary habitability also considers a range of other factors in determining the suitability of a planet for hosting life. The rogue planets in the Milky Way possibly number in the billions, the convention for designating exoplanets is an extension of the system used for designating multiple-star systems as adopted by the International Astronomical Union. For exoplanets orbiting a star, the designation is normally formed by taking the name or, more commonly, designation of its parent star. The first planet discovered in a system is given the designation b, if several planets in the same system are discovered at the same time, the closest one to the star gets the next letter, followed by the other planets in order of orbital size. A provisional IAU-sanctioned standard exists to accommodate the designation of circumbinary planets, a limited number of exoplanets have IAU-sanctioned proper names. Various detection claims made in the century were rejected by astronomers. The first scientific detection of an exoplanet began in 1988, However, the first confirmed detection came in 1992, with the discovery of several terrestrial-mass planets orbiting the pulsar PSR B1257+12. The first confirmation of an exoplanet orbiting a star was made in 1995. Some exoplanets have been imaged directly by telescopes, but the vast majority have been detected through indirect methods, such as the transit method, in the eighteenth century the same possibility was mentioned by Isaac Newton in the General Scholium that concludes his Principia. Claims of exoplanet detections have been made since the nineteenth century, some of the earliest involve the binary star 70 Ophiuchi. In 1855 William Stephen Jacob at the East India Companys Madras Observatory reported that orbital anomalies made it highly probable that there was a body in this system
14.
Star
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A star is a luminous sphere of plasma held together by its own gravity. The nearest star to Earth is the Sun, many other stars are visible to the naked eye from Earth during the night, appearing as a multitude of fixed luminous points in the sky due to their immense distance from Earth. Historically, the most prominent stars were grouped into constellations and asterisms, astronomers have assembled star catalogues that identify the known stars and provide standardized stellar designations. However, most of the stars in the Universe, including all stars outside our galaxy, indeed, most are invisible from Earth even through the most powerful telescopes. Almost all naturally occurring elements heavier than helium are created by stellar nucleosynthesis during the stars lifetime, near the end of its life, a star can also contain degenerate matter. Astronomers can determine the mass, age, metallicity, and many properties of a star by observing its motion through space, its luminosity. The total mass of a star is the factor that determines its evolution. Other characteristics of a star, including diameter and temperature, change over its life, while the environment affects its rotation. A plot of the temperature of stars against their luminosities produces a plot known as a Hertzsprung–Russell diagram. Plotting a particular star on that allows the age and evolutionary state of that star to be determined. A stars life begins with the collapse of a gaseous nebula of material composed primarily of hydrogen, along with helium. When the stellar core is sufficiently dense, hydrogen becomes steadily converted into helium through nuclear fusion, the remainder of the stars interior carries energy away from the core through a combination of radiative and convective heat transfer processes. The stars internal pressure prevents it from collapsing further under its own gravity, a star with mass greater than 0.4 times the Suns will expand to become a red giant when the hydrogen fuel in its core is exhausted. In some cases, it will fuse heavier elements at the core or in shells around the core, as the star expands it throws a part of its mass, enriched with those heavier elements, into the interstellar environment, to be recycled later as new stars. Meanwhile, the core becomes a remnant, a white dwarf. Binary and multi-star systems consist of two or more stars that are bound and generally move around each other in stable orbits. When two such stars have a close orbit, their gravitational interaction can have a significant impact on their evolution. Stars can form part of a much larger gravitationally bound structure, historically, stars have been important to civilizations throughout the world
15.
Trans-Neptunian object
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A trans-Neptunian object is any minor planet in the Solar System that orbits the Sun at a greater average distance than Neptune,30 astronomical units. Twelve minor planets with a semi-major axis greater than 150 AU and perihelion greater than 30 AU are known, the first trans-Neptunian object to be discovered was Pluto in 1930. It took until 1992 to discover a second trans-Neptunian object orbiting the Sun directly,1992 QB1, as of February 2017 over 2,300 trans-Neptunian objects appear on the Minor Planet Centers List of Transneptunian Objects. Of these TNOs,2,000 have a perihelion farther out than Neptune, as of November 2016,242 of these have their orbits well-enough determined that they have been given a permanent minor planet designation. The largest known object is Pluto, followed by Eris,2007 OR10, Makemake. The Kuiper belt, scattered disk, and Oort cloud are three divisions of this volume of space, though treatments vary and a few objects such as Sedna do not fit easily into any division. The orbit of each of the planets is slightly affected by the influences of the other planets. Discrepancies in the early 1900s between the observed and expected orbits of Uranus and Neptune suggested that there were one or more additional planets beyond Neptune, the search for these led to the discovery of Pluto in February 1930, which was too small to explain the discrepancies. Revised estimates of Neptunes mass from the Voyager 2 flyby in 1989 showed that the problem was spurious, Pluto was easiest to find because it has the highest apparent magnitude of all known trans-Neptunian objects. It also has an inclination to the ecliptic than most other large TNOs. After Plutos discovery, American astronomer Clyde Tombaugh continued searching for years for similar objects. For a long time, no one searched for other TNOs as it was believed that Pluto. Only after the 1992 discovery of a second TNO,1992 QB1, a broad strip of the sky around the ecliptic was photographed and digitally evaluated for slowly moving objects. Hundreds of TNOs were found, with diameters in the range of 50 to 2,500 kilometers, Pluto and Eris were eventually classified as dwarf planets by the International Astronomical Union. Kuiper belt objects are classified into the following two groups, Resonant objects are locked in an orbital resonance with Neptune. Objects with a 1,2 resonance are called twotinos, and objects with a 2,3 resonance are called plutinos, after their most prominent member, classical Kuiper belt objects have no such resonance, moving on almost circular orbits, unperturbed by Neptune. Examples are 1992 QB1,50000 Quaoar and Makemake, the scattered disc contains objects farther from the Sun, usually with very irregular orbits. A typical example is the most massive known TNO, Eris, scattered-extended —Scattered-extended objects have a Tisserand parameter greater than 3 and have a time-averaged eccentricity greater than 0
16.
Eris (dwarf planet)
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Eris is the most massive and second-largest dwarf planet known in the Solar System. It is also the known body directly orbiting the Sun. It is measured to be 2,326 ±12 kilometers in diameter, Eris is 27% more massive than dwarf planet Pluto, though Pluto is slightly larger by volume. Eris mass is about 0. 27% of the Earths mass, Eris was discovered in January 2005 by a Palomar Observatory-based team led by Mike Brown, and its identity was verified later that year. It is an object and a member of a high-eccentricity population known as the scattered disk. It has one moon, Dysnomia. As of February 2016, its distance from the Sun is 96.3 astronomical units, because Eris appeared to be larger than Pluto, NASA initially described it as the Solar Systems tenth planet. This, along with the prospect of other objects of similar size being discovered in the future, motivated the International Astronomical Union to define the term planet for the first time. Observations of an occultation by Eris in 2010 showed that its diameter was 2,326 ±12 kilometers, very slightly less than Pluto. Eris was discovered by the team of Mike Brown, Chad Trujillo, the discovery was announced on July 29,2005, the same day as Makemake and two days after Haumea, due in part to events that would later lead to controversy about Haumea. Routine observations were taken by the team on October 21,2003, in January 2005, the re-analysis revealed Eriss slow motion against the background stars. Follow-up observations were carried out to make a preliminary determination of Eriss orbit. More observations released in October 2005 revealed that Eris has a moon, observations of Dysnomias orbit permitted scientists to determine the mass of Eris, which in June 2007 they calculated to be ×1022 kg, 27%±2% greater than Plutos. Eris is named after the Greek goddess Eris, a personification of strife, the regular adjectival form of Eris is Eridian. As a result, for a time the object known to the wider public as Xena. Xena was a name used internally by the discovery team. It was inspired by the character of the television series Xena. The discovery team had saved the nickname Xena for the first body they discovered that was larger than Pluto
17.
Pluto
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Pluto is a dwarf planet in the Kuiper belt, a ring of bodies beyond Neptune. It was the first Kuiper belt object to be discovered, Pluto was discovered by Clyde Tombaugh in 1930 and was originally considered to be the ninth planet from the Sun. After 1992, its planethood was questioned following the discovery of objects of similar size in the Kuiper belt. In 2005, Eris, which is 27% more massive than Pluto, was discovered and this led the International Astronomical Union to define the term planet formally in 2006, during their 26th General Assembly. That definition excluded Pluto and reclassified it as a dwarf planet, Pluto is the largest and second-most-massive known dwarf planet in the Solar System and the ninth-largest and tenth-most-massive known object directly orbiting the Sun. It is the largest known trans-Neptunian object by volume but is less massive than Eris, like other Kuiper belt objects, Pluto is primarily made of ice and rock and is relatively small—about one-sixth the mass of the Moon and one-third its volume. It has an eccentric and inclined orbit during which it ranges from 30 to 49 astronomical units or AU from the Sun. This means that Pluto periodically comes closer to the Sun than Neptune, light from the Sun takes about 5.5 hours to reach Pluto at its average distance. Pluto has five moons, Charon, Styx, Nix, Kerberos. Pluto and Charon are sometimes considered a system because the barycenter of their orbits does not lie within either body. The IAU has not formalized a definition for binary dwarf planets, on July 14,2015, the New Horizons spacecraft became the first spacecraft to fly by Pluto. During its brief flyby, New Horizons made detailed measurements and observations of Pluto, on October 25,2016, at 05,48 pm ET, the last bit of data was received from New Horizons from its close encounter with Pluto on July 14,2015. In the 1840s, Urbain Le Verrier used Newtonian mechanics to predict the position of the then-undiscovered planet Neptune after analysing perturbations in the orbit of Uranus. Subsequent observations of Neptune in the late 19th century led astronomers to speculate that Uranuss orbit was being disturbed by another planet besides Neptune, by 1909, Lowell and William H. Pickering had suggested several possible celestial coordinates for such a planet. Lowell and his observatory conducted his search until his death in 1916, unknown to Lowell, his surveys had captured two faint images of Pluto on March 19 and April 7,1915, but they were not recognized for what they were. There are fourteen other known prediscovery observations, with the oldest made by the Yerkes Observatory on August 20,1909. Percivals widow, Constance Lowell, entered into a legal battle with the Lowell Observatory over her late husbands legacy. Tombaughs task was to image the night sky in pairs of photographs, then examine each pair
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International Astronomical Union
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The International Astronomical Union is an international association of professional astronomers, at the PhD level and beyond, active in professional research and education in astronomy. Among other activities, it acts as the recognized authority for assigning designations to celestial bodies. The IAU is a member of the International Council for Science and its main objective is to promote and safeguard the science of astronomy in all its aspects through international cooperation. The IAU maintains friendly relations with organizations that include amateur astronomers in their membership, the IAU has its head office on the second floor of the Institut dAstrophysique de Paris in the 14th arrondissement of Paris. The IAU is also responsible for the system of astronomical telegrams which are produced and distributed on its behalf by the Central Bureau for Astronomical Telegrams, the Minor Planet Center also operates under the IAU, and is a clearinghouse for all non-planetary or non-moon bodies in the Solar System. The Working Group for Meteor Shower Nomenclature and the Meteor Data Center coordinate the nomenclature of meteor showers, the IAU was founded on July 28,1919, at the Constitutive Assembly of the International Research Council held in Brussels, Belgium. The 7 initial member states were Belgium, Canada, France, Great Britain, Greece, Japan, the first executive committee consisted of Benjamin Baillaud, Alfred Fowler, and four vice presidents, William Campbell, Frank Dyson, Georges Lecointe, and Annibale Riccò. Thirty-two Commissions were appointed at the Brussels meeting and focused on topics ranging from relativity to minor planets, the reports of these 32 Commissions formed the main substance of the first General Assembly, which took place in Rome, Italy, May 2–10,1922. By the end of the first General Assembly, ten nations had joined the Union. Although the Union was officially formed eight months after the end of World War I, the first 50 years of the Unions history are well documented. Subsequent history is recorded in the form of reminiscences of past IAU Presidents, twelve of the fourteen past General Secretaries in the period 1964-2006 contributed their recollections of the Unions history in IAU Information Bulletin No.100. Six past IAU Presidents in the period 1976–2003 also contributed their recollections in IAU Information Bulletin No.104, the IAU includes a total of 12,664 individual members who are professional astronomers from 96 countries worldwide. 83% of all members are male, while 17% are female, among them the unions current president. Membership also includes 79 national members, professional astronomical communities representing their countrys affiliation with the IAU, the sovereign body of the IAU is its General Assembly, which comprises all members. The Assembly determines IAU policy, approves the Statutes and By-Laws of the Union, the right to vote on matters brought before the Assembly varies according to the type of business under discussion. On budget matters, votes are weighted according to the subscription levels of the national members. A second category vote requires a turnout of at least two-thirds of national members in order to be valid, an absolute majority is sufficient for approval in any vote, except for Statute revision which requires a two-thirds majority. An equality of votes is resolved by the vote of the President of the Union, since 1922, the IAU General Assembly meets every three years, with the exception of the period between 1938 and 1948, due to World War II
19.
2006 definition of planet
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A non-satellite body fulfilling only the first two of these criteria is classified as a dwarf planet. According to the IAU, planets and dwarf planets are two classes of objects. A non-satellite body fulfilling only the first criterion is termed a small Solar System body, an alternate proposal included dwarf planets as a subcategory of planets, but IAU members voted against this proposal. The definition was a one, and has drawn both support and criticism from different astronomers, but has remained in use. According to this definition, there are eight planets in the Solar System. The definition distinguishes planets from smaller bodies and is not useful outside the Solar System, extrasolar planets, or exoplanets, are covered separately under a complementary 2003 draft guideline for the definition of planets, which distinguishes them from dwarf stars, which are larger. Before the discoveries of the early 21st century, astronomers had no real need for a definition for planets. With the discovery of Pluto in 1930, astronomers considered the Solar System to have nine planets, along with thousands of bodies such as asteroids. Pluto was thought to be larger than Mercury, in 1978, the discovery of Plutos moon Charon radically changed this picture. By measuring Charons orbital period, astronomers could accurately calculate Plutos mass for the first time, in the 1990s, astronomers began finding other objects at least as far away as Pluto, now known as Kuiper Belt objects, or KBOs. Many of these some of Plutos key orbital characteristics and are now called plutinos. Pluto came to be seen as the largest member of a new class of objects, Plutos eccentric and inclined orbit, while very unusual for a planet in the Solar System, fits in well with the other KBOs. New York Citys newly renovated Hayden Planetarium did not include Pluto in its exhibit of the planets when it reopened as the Rose Center for Earth, astronomers also knew that more objects as large as Pluto would be discovered, and the number of planets would start growing quickly. They were also concerned about the classification of planets in other planetary systems, in 2006, the matter came to a head with the first measurement of the size of 2003 UB313. That measurement had showed Eris to appear to be larger than Pluto. The process of new discoveries spurring a contentious refinement of Plutos categorization echoed a debate in the 19th century that began with the discovery of Ceres on January 1,1801, astronomers immediately declared the tiny object to be the missing planet between Mars and Jupiter. Within four years, however, the discovery of two objects with comparable sizes and orbits had cast doubt on this new thinking. By 1851, the number of planets had grown to 23, astronomers began cataloguing them separately and began calling them asteroids instead of planets
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Prague
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Prague is the capital and largest city of the Czech Republic. It is the 14th largest city in the European Union and it is also the historical capital of Bohemia. Situated in the north-west of the country on the Vltava river, the city has a temperate climate, with warm summers and chilly winters. Prague has been a political, cultural, and economic centre of central Europe with waxing and waning fortunes during its history and it was an important city to the Habsburg Monarchy and its Austro-Hungarian Empire. Prague is home to a number of cultural attractions, many of which survived the violence. Main attractions include the Prague Castle, the Charles Bridge, Old Town Square with the Prague astronomical clock, since 1992, the extensive historic centre of Prague has been included in the UNESCO list of World Heritage Sites. The city has more than ten major museums, along with theatres, galleries, cinemas. An extensive modern public transportation system connects the city, also, it is home to a wide range of public and private schools, including Charles University in Prague, the oldest university in Central Europe. Prague is classified as an Alpha- global city according to GaWC studies, Prague ranked sixth in the Tripadvisor world list of best destinations in 2016. Its rich history makes it a popular tourist destination, and the city more than 6.4 million international visitors annually. Prague is the fifth most visited European city after London, Paris, Istanbul, the region was settled as early as the Paleolithic age. In the last century BC, the Celts were slowly driven away by Germanic tribes, around the area where present-day Prague stands, the 2nd century map of Ptolemaios mentioned a Germanic city called Casurgis. In the following century, the Czech tribes built several fortified settlements in the area, most notably in Levý Hradec, Butovice and in the Šárka valley. The construction of what came to be known as the Prague Castle began near the end of the 9th century, the first masonry under Prague Castle dates from the year 885 at the latest. The other prominent Prague fort, the Přemyslid fort Vyšehrad, was founded in the 10th century, Prague Castle is dominated by the cathedral, which was founded in 1344, but completed in the 20th century. The legendary origins of Prague attribute its foundation to the 8th century Czech duchess and prophetess Libuše and her husband, Přemysl, legend says that Libuše came out on a rocky cliff high above the Vltava and prophesied, I see a great city whose glory will touch the stars. She ordered a castle and a town called Praha to be built on the site, a 17th century Jewish chronicler David Solomon Ganz, citing Cyriacus Spangenberg, claimed that the city was founded as Boihaem in c.1306 BC by an ancient king, Boyya. The region became the seat of the dukes, and later kings of Bohemia, under Roman Emperor Otto II the area became a bishopric in 973
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Czech Republic
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The Czech Republic, also known as Czechia, is a nation state in Central Europe bordered by Germany to the west, Austria to the south, Slovakia to the east and Poland to the northeast. The Czech Republic covers an area of 78,866 square kilometres with mostly temperate continental climate and it is a unitary parliamentary republic, has 10.5 million inhabitants and the capital and largest city is Prague, with over 1.2 million residents. The Czech Republic includes the territories of Bohemia, Moravia. The Czech state was formed in the late 9th century as the Duchy of Bohemia under the Great Moravian Empire, after the fall of the Empire in 907, the centre of power transferred from Moravia to Bohemia under the Přemyslid dynasty. In 1002, the duchy was formally recognized as part of the Holy Roman Empire, becoming the Kingdom of Bohemia in 1198 and reaching its greatest territorial extent in the 14th century. Following the Battle of Mohács in 1526, the whole Crown of Bohemia was gradually integrated into the Habsburg Monarchy alongside the Archduchy of Austria, the Protestant Bohemian Revolt against the Catholic Habsburgs led to the Thirty Years War. After the Battle of the White Mountain, the Habsburgs consolidated their rule, reimposed Roman Catholicism, the Czech part of Czechoslovakia was occupied by Germany in World War II, and was liberated in 1945 by the armies of the Soviet Union and the United States. The Czech country lost the majority of its German-speaking inhabitants after they were expelled following the war, the Communist Party of Czechoslovakia won the 1946 elections. Following the 1948 coup détat, Czechoslovakia became a one-party communist state under Soviet influence, in 1968, increasing dissatisfaction with the regime culminated in a reform movement known as the Prague Spring, which ended in a Soviet-led invasion. Czechoslovakia remained occupied until the 1989 Velvet Revolution, when the communist regime collapsed, on 6 March 1990, the Czech Socialistic Republic was renamed to the Czech Republic. On 1 January 1993, Czechoslovakia peacefully dissolved, with its constituent states becoming the independent states of the Czech Republic and the Slovak Republic. The Czech Republic joined NATO in 1999 and the European Union in 2004, it is a member of the United Nations, the OECD, the OSCE, and it is a developed country with an advanced, high income economy and high living standards. The UNDP ranks the country 14th in inequality-adjusted human development, the Czech Republic also ranks as the 6th most peaceful country, while achieving strong performance in democratic governance. It has the lowest unemployment rate in the European Union, the traditional English name Bohemia derives from Latin Boiohaemum, which means home of the Boii. The current name comes from the endonym Čech, spelled Cžech until the reform in 1842. The name comes from the Slavic tribe and, according to legend, their leader Čech, the etymology of the word Čech can be traced back to the Proto-Slavic root *čel-, meaning member of the people, kinsman, thus making it cognate to the Czech word člověk. The country has traditionally divided into three lands, namely Bohemia in the west, Moravia in the southeast, and Czech Silesia in the northeast. Following the dissolution of Czechoslovakia at the end of 1992, the Czech part of the former nation found itself without a common single-word geographical name in English, the name Czechia /ˈtʃɛkiə/ was recommended by the Czech Ministry of Foreign Affairs
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Mass
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In physics, mass is a property of a physical body. It is the measure of a resistance to acceleration when a net force is applied. It also determines the strength of its gravitational attraction to other bodies. The basic SI unit of mass is the kilogram, Mass is not the same as weight, even though mass is often determined by measuring the objects weight using a spring scale, rather than comparing it directly with known masses. An object on the Moon would weigh less than it does on Earth because of the lower gravity and this is because weight is a force, while mass is the property that determines the strength of this force. In Newtonian physics, mass can be generalized as the amount of matter in an object, however, at very high speeds, special relativity postulates that energy is an additional source of mass. Thus, any body having mass has an equivalent amount of energy. In addition, matter is a defined term in science. There are several distinct phenomena which can be used to measure mass, active gravitational mass measures the gravitational force exerted by an object. Passive gravitational mass measures the force exerted on an object in a known gravitational field. The mass of an object determines its acceleration in the presence of an applied force, according to Newtons second law of motion, if a body of fixed mass m is subjected to a single force F, its acceleration a is given by F/m. A bodys mass also determines the degree to which it generates or is affected by a gravitational field and this is sometimes referred to as gravitational mass. The standard International System of Units unit of mass is the kilogram, the kilogram is 1000 grams, first defined in 1795 as one cubic decimeter of water at the melting point of ice. Then in 1889, the kilogram was redefined as the mass of the prototype kilogram. As of January 2013, there are proposals for redefining the kilogram yet again. In this context, the mass has units of eV/c2, the electronvolt and its multiples, such as the MeV, are commonly used in particle physics. The atomic mass unit is 1/12 of the mass of a carbon-12 atom, the atomic mass unit is convenient for expressing the masses of atoms and molecules. Outside the SI system, other units of mass include, the slug is an Imperial unit of mass, the pound is a unit of both mass and force, used mainly in the United States
23.
Hydrostatic equilibrium
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In fluid mechanics, a fluid is said to be in hydrostatic equilibrium or hydrostatic balance when it is at rest, or when the flow velocity at each point is constant over time. This occurs when external forces such as gravity are balanced by a pressure gradient force, hydrostatic equilibrium is the current distinguishing criterion between dwarf planets and small Solar System bodies, and has other roles in astrophysics and planetary geology. This qualification typically means that the object is symmetrically rounded into a spheroid or ellipsoid shape, there are 31 observationally confirmed such objects, sometimes called planemos, in the Solar System, seven more that are virtually certain, and a hundred or so more that are likely. Newtons laws of state that a volume of a fluid that is not in motion or that is in a state of constant velocity must have zero net force on it. This means the sum of the forces in a given direction must be opposed by a sum of forces in the opposite direction. This force balance is called a hydrostatic equilibrium, the fluid can be split into a large number of cuboid volume elements, by considering a single element, the action of the fluid can be derived. If the density is ρ, the volume is V and g the standard gravity, then, the volume of this cuboid is equal to the area of the top or bottom, times the height — the formula for finding the volume of a cube. This sum equals zero if the velocity is constant. Dividing by A,0 = P b o t t o m − P t o p − ρ ⋅ g ⋅ h, or, P t o p − P b o t t o m = − ρ ⋅ g ⋅ h. Ptop − Pbottom is a change in pressure, and h is the height of the volume change in the distance above the ground. By saying these changes are small, the equation can be written in differential form. Density changes with pressure, and gravity changes with height, so the equation would be, d P = − ρ ⋅ g ⋅ d h. Note finally that this last equation can be derived by solving the three-dimensional Navier–Stokes equations for the situation where u = v = ∂ p ∂ x = ∂ p ∂ y =0. Then the only equation is the z -equation, which now reads ∂ p ∂ z + ρ g =0. Thus, hydrostatic balance can be regarded as a simple equilibrium solution of the Navier–Stokes equations. M is a foliation of spheres weighted by the mass density ρ, the hydrostatic equilibrium pertains to hydrostatics and the principles of equilibrium of fluids. A hydrostatic balance is a balance for weighing substances in water. Hydrostatic balance allows the discovery of their specific gravities, in any given layer of a star, there is a hydrostatic equilibrium between the outward thermal pressure from below and the weight of the material above pressing inward
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Clearing the neighbourhood
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Clearing the neighbourhood around its orbit is a criterion for a celestial body to be considered a planet in the Solar System. This was one of the three criteria adopted by the International Astronomical Union in its 2006 definition of planet, in 2015, a proposal was made to use the criterion in extending the definition to exoplanets. A large body that meets the criteria for a planet but has not cleared its neighbourhood is classified as a dwarf planet. This includes Pluto, which is constrained in its orbit by the gravity of Neptune, the phrase may be derived from a paper presented to the general assembly of the IAU in 2000 by Alan Stern and Harold F. Levison. Steven Soter prefers to use the term dynamical dominance and Jean-Luc Margot notes that such language seems less prone to misinterpretation, the naming process for Eris stalled after the announcement of its discovery in 2005, pending clarification of this first step. The phrase refers to an orbiting body sweeping out its orbital region over time, as a consequence it does not then share its orbital region with other bodies of significant size, except for its own satellites, or other bodies governed by its own gravitational influence. In their paper, Stern and Levison sought an algorithm to determine which planetary bodies control the region surrounding them. They defined Λ, a measure of an ability to scatter smaller masses out of its orbital region over a period of time equal to the age of the Universe. In the domain of the solar disc, there is little variation in the average values of k for small bodies at a particular distance from the Sun. If Λ >1, then the body will likely clear out the bodies in its orbital zone. The überplanets are the eight most massive solar orbiters, and the unterplanets are the rest, steven Soter proposed an observationally based measure µ, which he called the planetary discriminant, to separate bodies orbiting stars into planets and non-planets. Per Soter, two bodies are defined to share an orbital zone if their orbits cross a common radial distance from the primary, µ is then calculated by dividing the mass of the candidate body by the total mass of the other objects that share its orbital zone. It is a measure of the degree of cleanliness of the orbital zone. Soter proposed that if µ >100, then the body be regarded as a planet. Astronomer Jean-Luc Margot has proposed a discriminant, Π, that can categorise a body based only on its own mass, its semi-major axis, and its stars mass. Like Stern–Levisons Λ, Π is a measure of the ability of the body to clear its orbit, Π is based on properties that are feasibly determinable even for exoplanetary bodies, unlike Soters µ, which requires an accurate census of the orbital zone. Π depends on the extent of clearing desired and the required to do so. Margot selected an extent of 23 times the Hill radius, then, in the mentioned units and a main-sequence lifetime of 10 billion years, k =807
25.
Geocentric model
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In astronomy, the geocentric model is a superseded description of the universe with the Earth at the center. Under the geocentric model, the Sun, Moon, stars, the geocentric model served as the predominant description of the cosmos in many ancient civilizations, such as those of Aristotle and Ptolemy. Two observations supported the idea that the Earth was the center of the Universe, first, the Sun appears to revolve around the Earth once per day. While the Moon and the planets have their own motions, they appear to revolve around the Earth about once per day. The stars appeared to be on a sphere, rotating once each day along an axis through the north and south geographic poles of the Earth. Second, the Earth does not seem to move from the perspective of an Earth-bound observer, it appears to be solid, stable, Ancient Greek, ancient Roman and medieval philosophers usually combined the geocentric model with a spherical Earth. It is not the same as the older flat Earth model implied in some mythology, the ancient Jewish Babylonian uranography pictured a flat Earth with a dome-shaped rigid canopy named firmament placed over it. The astronomical predictions of Ptolemys geocentric model were used to prepare astrological and astronomical charts for over 1500 years. The geocentric model held sway into the modern age, but from the late 16th century onward, it was gradually superseded by the Heliocentric model of Copernicus, Galileo. There was much resistance to the transition between these two theories, christian theologians were reluctant to reject a theory that agreed with Bible passages. Others felt a new, unknown theory could not subvert an accepted consensus for geocentrism, the geocentric model entered Greek astronomy and philosophy at an early point, it can be found in Pre-Socratic philosophy. In the 6th century BC, Anaximander proposed a cosmology with the Earth shaped like a section of a pillar, the Sun, Moon, and planets were holes in invisible wheels surrounding the Earth, through the holes, humans could see concealed fire. About the same time, the Pythagoreans thought that the Earth was a sphere, later these views were combined, so most educated Greeks from the 4th century BC on thought that the Earth was a sphere at the center of the universe. In the 4th century BC, two influential Greek philosophers, Plato and his student Aristotle, wrote works based on the geocentric model, according to Plato, the Earth was a sphere, stationary at the center of the universe. In his Myth of Er, a section of the Republic, Plato describes the cosmos as the Spindle of Necessity, attended by the Sirens and these spheres, known as crystalline spheres, all moved at different uniform speeds to create the revolution of bodies around the Earth. They were composed of a substance called aether. Aristotle believed that the moon was in the innermost sphere and therefore touches the realm of Earth, causing the dark spots and he further described his system by explaining the natural tendencies of the terrestrial elements, Earth, water, fire, air, as well as celestial aether. His system held that Earth was the heaviest element, with the strongest movement towards the center, the tendency of air and fire, on the other hand, was to move upwards, away from the center, with fire being lighter than air
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Heliocentrism
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Heliocentrism is the astronomical model in which the Earth and planets revolve around the Sun at the center of the Solar System. Historically, Heliocentrism was opposed to geocentrism, which placed the Earth at the center, in the following century, Johannes Kepler elaborated upon and expanded this model to include elliptical orbits, and Galileo Galilei presented supporting observations made using a telescope. With the observations of William Herschel, Friedrich Bessel, and others, astronomers realized that the sun, the Ptolemaic system was a sophisticated astronomical system that managed to calculate the positions for the planets to a fair degree of accuracy. However, he rejected the idea of a spinning earth as absurd as he believed it would create huge winds and his planetary hypotheses were sufficiently real that the distances of moon, sun, planets and stars could be determined by treating orbits celestial spheres as contiguous realities. This system postulated the existence of a counter-earth collinear with the Earth and central fire, the Sun revolved around the central fire once a year, and the stars were stationary. The Earth maintained the same face towards the central fire. Kepler gave an explanation of the Pythagoreans central fire as the Sun. Heraclides of Pontus said that the rotation of the Earth explained the apparent daily motion of the celestial sphere and it used to be thought that he believed Mercury and Venus to revolve around the Sun, which in turn revolves around the Earth. Macrobius Ambrosius Theodosius later described this as the Egyptian System, stating that it did not escape the skill of the Egyptians, the first person known to have proposed a heliocentric system, however, was Aristarchus of Samos. Like Eratosthenes, Aristarchus calculated the size of the Earth, and measured the size and distance of the Moon and Sun, from his estimates, he concluded that the Sun was six to seven times wider than the Earth and thus hundreds of times more voluminous. His writings on the system are lost, but some information is known from surviving descriptions and critical commentary by his contemporaries. This is the account as you have heard from astronomers. The stars are in fact much farther away than the distance that was assumed in ancient times. Archimedes says that Aristarchus made the stars distance larger, suggesting that he was answering the natural objection that Heliocentrism requires stellar parallactic oscillations and he apparently agreed to the point but placed the stars so distant as to make the parallactic motion invisibly minuscule. Thus Heliocentrism opened the way for realization that the universe was larger than the geocentrists taught, according to one of Plutarchs characters in the dialogue, the philosopher Cleanthes had held that Aristarchus should be charged with impiety for moving the hearth of the world. Since Plutarch mentions the followers of Aristarchus in passing, it is likely there were other astronomers in the Classical period who also espoused Heliocentrism. Seleucus adopted the system of Aristarchus and is said to have proved the heliocentric theory. He may have used trigonometric methods that were available in his time
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Celestial spheres
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The celestial spheres, or celestial orbs, were the fundamental entities of the cosmological models developed by Plato, Eudoxus, Aristotle, Ptolemy, Copernicus and others. Since it was believed that the stars did not change their positions relative to one another. In modern thought, the orbits of the planets are viewed as the paths of those planets through mostly empty space, when scholars applied Ptolemys epicycles, they presumed that each planetary sphere was exactly thick enough to accommodate them. In Greek antiquity the ideas of celestial spheres and rings first appeared in the cosmology of Anaximander in the early 6th century BC, all these wheel rims had originally been formed out of an original sphere of fire wholly encompassing the Earth, which had disintegrated into many individual rings. Hence, in Anaximanderss cosmogony, in the beginning was the sphere, out of which celestial rings were formed, as viewed from the Earth, the ring of the Sun was highest, that of the Moon was lower, and the sphere of the stars was lowest. Following Anaximander, his pupil Anaximenes held that the stars, Sun, Moon, and planets are all made of fire. But whilst the stars are fastened on a crystal sphere like nails or studs, the Sun, Moon, and planets. And unlike Anaximander, he relegated the fixed stars to the region most distant from the Earth, after Anaximenes, Pythagoras, Xenophanes and Parmenides all held that the universe was spherical. And much later in the fourth century BC Platos Timaeus proposed that the body of the cosmos was made in the most perfect and uniform shape, but it posited that the planets were spherical bodies set in rotating bands or rings rather than wheel rims as in Anaximanders cosmology. Each planet is attached to the innermost of its own set of spheres. In his Metaphysics, Aristotle developed a cosmology of spheres. Aristotle considers that these spheres are made of a fifth element. Each of these concentric spheres is moved by its own god—an unchanging divine unmoved mover, by using eccentrics and epicycles, his geometrical model achieved greater mathematical detail and predictive accuracy than had been exhibited by earlier concentric spherical models of the cosmos. In Ptolemys physical model, each planet is contained in two or more spheres, but in Book 2 of his Planetary Hypotheses Ptolemy depicted thick circular slices rather than spheres as in its Book 1. The planetary spheres were arranged outwards from the spherical, stationary Earth at the centre of the universe in order, the spheres of the Moon, Mercury, Venus, Sun, Mars, Jupiter. In more detailed models the seven planetary spheres contained other secondary spheres within them, in antiquity the order of the lower planets was not universally agreed. Plato and his followers ordered them Moon, Sun, Mercury, Venus, a series of astronomers, beginning with the Muslim astronomer al-Farghānī, used the Ptolemaic model of nesting spheres to compute distances to the stars and planetary spheres. Al-Farghānīs distance to the stars was 20,110 Earth radii which, on the assumption that the radius of the Earth was 3,250 miles, came to 65,357,500 miles
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Classical planet
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The word planet comes from the Greek word πλανήτης, planētēs wanderer, expressing the fact that these objects move across the celestial sphere relative to the fixed stars. In the original papyri of these Greek horoscopes, there are found a circle with one ray for the Sun, the written symbols for Mercury, Venus, Jupiter, and Saturn have been traced to forms found in late Greek papyri. The symbols for Jupiter and Saturn are identified as monograms of the letters of the corresponding Greek names. A. S. D. Maunder finds antecedents of the symbols in earlier sources. The modern sun symbol, pictured as a circle with a dot, the Ptolemaic system used in Greek astronomy placed the planets in order, closest to Earth to furthest, as the Moon, Mercury, Venus, Sun, Mars, Jupiter, and Saturn. In addition the day was divided into 7-hour intervals, each ruled by one of the planets, the first hour of each day was named after the ruling planet, giving rise to the names and order of the Roman seven-day week. The modern English days of the week were inherited from gods of the old Germanic Norse culture—Thursday = Thor, Friday = Frige. It can be correlated that the Norse gods were attributed to each Roman planet and its god, in alchemy, each classical planet was associated with one of the seven metals known to the classical world. As a result, the glyphs for the metal and associated planet coincide. Alchemists believed the other metals were variants of these seven. Astrology has used the concept of elements from antiquity up until the present day today. Most modern astrologers use the four classical elements extensively, and indeed they are viewed as a critical part of interpreting the astrological chart. Traditionally, each of the seven planets in the system as known to the ancients was associated with, held dominion over. Mercury and Venus are visible only in twilight hours because their orbits are interior to that of Earth, Venus is the third-brightest object in the sky and the most prominent planet. Mercury is more difficult to see due to its proximity to the Sun, lengthy twilight and an extremely low angle at maximum elongations make optical filters necessary to see Mercury from extreme polar locations. Mars is at its brightest when it is in opposition, which occurs approximately every twenty-five months, Jupiter and Saturn are the largest of the five planets, but are farther from the Sun, and therefore receive less sunlight. Nonetheless, Jupiter is often the next brightest object in the sky after Venus
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Plato
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Plato was a philosopher in Classical Greece and the founder of the Academy in Athens, the first institution of higher learning in the Western world. He is widely considered the most pivotal figure in the development of philosophy, unlike nearly all of his philosophical contemporaries, Platos entire work is believed to have survived intact for over 2,400 years. Along with his teacher, Socrates, and his most famous student, Aristotle, Plato laid the foundations of Western philosophy. Alfred North Whitehead once noted, the safest general characterization of the European philosophical tradition is that it consists of a series of footnotes to Plato. In addition to being a figure for Western science, philosophy. Friedrich Nietzsche, amongst other scholars, called Christianity, Platonism for the people, Plato was the innovator of the written dialogue and dialectic forms in philosophy, which originate with him. He was not the first thinker or writer to whom the word “philosopher” should be applied, few other authors in the history of Western philosophy approximate him in depth and range, perhaps only Aristotle, Aquinas and Kant would be generally agreed to be of the same rank. Due to a lack of surviving accounts, little is known about Platos early life, the philosopher came from one of the wealthiest and most politically active families in Athens. Ancient sources describe him as a bright though modest boy who excelled in his studies, the exact time and place of Platos birth are unknown, but it is certain that he belonged to an aristocratic and influential family. Based on ancient sources, most modern scholars believe that he was born in Athens or Aegina between 429 and 423 BCE. According to a tradition, reported by Diogenes Laertius, Ariston traced his descent from the king of Athens, Codrus. Platos mother was Perictione, whose family boasted of a relationship with the famous Athenian lawmaker, besides Plato himself, Ariston and Perictione had three other children, these were two sons, Adeimantus and Glaucon, and a daughter Potone, the mother of Speusippus. The brothers Adeimantus and Glaucon are mentioned in the Republic as sons of Ariston, and presumably brothers of Plato, but in a scenario in the Memorabilia, Xenophon confused the issue by presenting a Glaucon much younger than Plato. Then, at twenty-eight, Hermodorus says, went to Euclides in Megara, as Debra Nails argues, The text itself gives no reason to infer that Plato left immediately for Megara and implies the very opposite. Thus, Nails dates Platos birth to 424/423, another legend related that, when Plato was an infant, bees settled on his lips while he was sleeping, an augury of the sweetness of style in which he would discourse about philosophy. Ariston appears to have died in Platos childhood, although the dating of his death is difficult. Perictione then married Pyrilampes, her mothers brother, who had served many times as an ambassador to the Persian court and was a friend of Pericles, Pyrilampes had a son from a previous marriage, Demus, who was famous for his beauty. Perictione gave birth to Pyrilampes second son, Antiphon, the half-brother of Plato and these and other references suggest a considerable amount of family pride and enable us to reconstruct Platos family tree
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Aristarchus of Samos
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Aristarchus of Samos was an ancient Greek astronomer and mathematician who presented the first known model that placed the Sun at the center of the known universe with the Earth revolving around it. He was influenced by Philolaus of Croton, but he identified the central fire with the Sun, like Anaxagoras before him, he suspected that the stars were just other bodies like the Sun, albeit further away from Earth. His astronomical ideas were rejected in favor of the geocentric theories of Aristotle and Ptolemy. Nicolaus Copernicus had attributed the theory to Aristarchus. This is the account as you have heard from astronomers. Since stellar parallax is only detectable with telescopes, his accurate speculation was unprovable at the time and it is a common misconception that the heliocentric view was held as sacrilegious by the contemporaries of Aristarchus. This is due to Gilles Ménages translation of a passage from Plutarchs On the Apparent Face in the Orb of the Moon. Plutarch reported that Cleanthes as a worshipper of the Sun and opponent to the model, was jokingly told by Aristarchus that he should be charged with impiety. Gilles Ménage, shortly after the trials of Galileo and Giordano Bruno, amended an accusative with a nominative, the resulting misconception of an isolated and persecuted Aristarchus is still transmitted today. In his Naturalis Historia, Pliny the Elder later wondered whether errors in the predictions about the heavens could be attributed to a displacement of the Earth from its central position. Pliny and Seneca referred to planets retrograde motion as an apparent phenomenon, still, no stellar parallax was observed, and Plato, Aristotle and Ptolemy preferred the geocentric model, which was held as true throughout the Middle Ages. The heliocentric theory was revived by Copernicus, after which Johannes Kepler described planetary motions with greater accuracy with his three laws, isaac Newton later gave a theoretical explanation based on laws of gravitational attraction and dynamics. The only surviving work attributed to Aristarchus, On the Sizes and Distances of the Sun. The discrepancy may come from a misinterpretation of what unit of measure was meant by a certain Greek term in Aristarchus text, Aristarchus claimed that at half moon, the angle between the Sun and Moon was 87°. He might have proposed 87° as a bound, since gauging the lunar terminators deviation from linearity to 1° accuracy is beyond the unaided human ocular limit. Aristarchus is known to have also studied light and vision, using correct geometry, but the insufficiently accurate 87° datum, Aristarchus concluded that the Sun was between 18 and 20 times farther away than the Moon. The implicit false solar parallax of slightly under 3° was used by astronomers up to and including Tycho Brahe, Aristarchus inequality Belief Perseverance Heath, Sir Thomas. Gomez, A. G. Aristarchos of Samos, the Polymath, online Galleries, History of Science Collections, University of Oklahoma Libraries High resolution images of works by Aristarchus of Samos in. jpg and. tiff format
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Mercury (planet)
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Mercury is the smallest and innermost planet in the Solar System. Its orbital period around the Sun of 88 days is the shortest of all the planets in the Solar System and it is named after the Roman deity Mercury, the messenger to the gods. Like Venus, Mercury orbits the Sun within Earths orbit as a planet, so it can only be seen visually in the morning or the evening sky. Also, like Venus and the Moon, the displays the complete range of phases as it moves around its orbit relative to Earth. Seen from Earth, this cycle of phases reoccurs approximately every 116 days, although Mercury can appear as a bright star-like object when viewed from Earth, its proximity to the Sun often makes it more difficult to see than Venus. Mercury is tidally or gravitationally locked with the Sun in a 3,2 resonance, as seen relative to the fixed stars, it rotates on its axis exactly three times for every two revolutions it makes around the Sun. As seen from the Sun, in a frame of reference that rotates with the orbital motion, an observer on Mercury would therefore see only one day every two years. Mercurys axis has the smallest tilt of any of the Solar Systems planets, at aphelion, Mercury is about 1.5 times as far from the Sun as it is at perihelion. Mercurys surface appears heavily cratered and is similar in appearance to the Moons, the polar regions are constantly below 180 K. The planet has no natural satellites. Mercury is one of four planets in the Solar System. It is the smallest planet in the Solar System, with a radius of 2,439.7 kilometres. Mercury is also smaller—albeit more massive—than the largest natural satellites in the Solar System, Ganymede, Mercury consists of approximately 70% metallic and 30% silicate material. Mercurys density is the second highest in the Solar System at 5.427 g/cm3, Mercurys density can be used to infer details of its inner structure. Although Earths high density results appreciably from gravitational compression, particularly at the core, Mercury is much smaller, therefore, for it to have such a high density, its core must be large and rich in iron. Geologists estimate that Mercurys core occupies about 55% of its volume, Research published in 2007 suggests that Mercury has a molten core. Surrounding the core is a 500–700 km mantle consisting of silicates, based on data from the Mariner 10 mission and Earth-based observation, Mercurys crust is estimated to be 35 km thick. One distinctive feature of Mercurys surface is the presence of narrow ridges
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Venus
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Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. It has the longest rotation period of any planet in the Solar System and it is named after the Roman goddess of love and beauty. It is the second-brightest natural object in the sky after the Moon, reaching an apparent magnitude of −4.6. Because Venus orbits within Earths orbit it is a planet and never appears to venture far from the Sun. Venus is a planet and is sometimes called Earths sister planet because of their similar size, mass, proximity to the Sun. It is radically different from Earth in other respects and it has the densest atmosphere of the four terrestrial planets, consisting of more than 96% carbon dioxide. The atmospheric pressure at the surface is 92 times that of Earth. Venus is by far the hottest planet in the Solar System, with a surface temperature of 735 K. Venus is shrouded by an layer of highly reflective clouds of sulfuric acid. It may have had water oceans in the past, but these would have vaporized as the temperature rose due to a greenhouse effect. The water has probably photodissociated, and the hydrogen has been swept into interplanetary space by the solar wind because of the lack of a planetary magnetic field. Venuss surface is a dry desertscape interspersed with rocks and is periodically resurfaced by volcanism. As one of the brightest objects in the sky, Venus has been a fixture in human culture for as long as records have existed. It has been sacred to gods of many cultures, and has been a prime inspiration for writers and poets as the morning star. Venus was the first planet to have its motions plotted across the sky, as the closest planet to Earth, Venus has been a prime target for early interplanetary exploration. It was the first planet beyond Earth visited by a spacecraft, Venuss thick clouds render observation of its surface impossible in visible light, and the first detailed maps did not emerge until the arrival of the Magellan orbiter in 1991. Plans have been proposed for rovers or more missions. Venus is one of the four planets in the Solar System
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Mars
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Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, after Mercury. Named after the Roman god of war, it is referred to as the Red Planet because the iron oxide prevalent on its surface gives it a reddish appearance. Mars is a planet with a thin atmosphere, having surface features reminiscent both of the impact craters of the Moon and the valleys, deserts, and polar ice caps of Earth. The rotational period and seasonal cycles of Mars are likewise similar to those of Earth, Mars is the site of Olympus Mons, the largest volcano and second-highest known mountain in the Solar System, and of Valles Marineris, one of the largest canyons in the Solar System. The smooth Borealis basin in the northern hemisphere covers 40% of the planet, Mars has two moons, Phobos and Deimos, which are small and irregularly shaped. These may be captured asteroids, similar to 5261 Eureka, a Mars trojan, there are ongoing investigations assessing the past habitability potential of Mars, as well as the possibility of extant life. Future astrobiology missions are planned, including the Mars 2020 and ExoMars rovers, liquid water cannot exist on the surface of Mars due to low atmospheric pressure, which is about 6⁄1000 that of the Earths, except at the lowest elevations for short periods. The two polar ice caps appear to be largely of water. The volume of ice in the south polar ice cap, if melted. On November 22,2016, NASA reported finding a large amount of ice in the Utopia Planitia region of Mars. The volume of water detected has been estimated to be equivalent to the volume of water in Lake Superior, Mars can easily be seen from Earth with the naked eye, as can its reddish coloring. Its apparent magnitude reaches −2.91, which is surpassed only by Jupiter, Venus, the Moon, optical ground-based telescopes are typically limited to resolving features about 300 kilometers across when Earth and Mars are closest because of Earths atmosphere. Mars is approximately half the diameter of Earth with an area only slightly less than the total area of Earths dry land. Mars is less dense than Earth, having about 15% of Earths volume and 11% of Earths mass, the red-orange appearance of the Martian surface is caused by iron oxide, or rust. It can look like butterscotch, other common colors include golden, brown, tan. Like Earth, Mars has differentiated into a metallic core overlaid by less dense materials. Current models of its interior imply a core with a radius of about 1,794 ±65 kilometers, consisting primarily of iron and this iron sulfide core is thought to be twice as rich in lighter elements than Earths. The core is surrounded by a mantle that formed many of the tectonic and volcanic features on the planet
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Jupiter
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Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a giant planet with a mass one-thousandth that of the Sun, Jupiter and Saturn are gas giants, the other two giant planets, Uranus and Neptune are ice giants. Jupiter has been known to astronomers since antiquity, the Romans named it after their god Jupiter. Jupiter is primarily composed of hydrogen with a quarter of its mass being helium and it may also have a rocky core of heavier elements, but like the other giant planets, Jupiter lacks a well-defined solid surface. Because of its rotation, the planets shape is that of an oblate spheroid. The outer atmosphere is visibly segregated into several bands at different latitudes, resulting in turbulence, a prominent result is the Great Red Spot, a giant storm that is known to have existed since at least the 17th century when it was first seen by telescope. Surrounding Jupiter is a faint planetary ring system and a powerful magnetosphere, Jupiter has at least 67 moons, including the four large Galilean moons discovered by Galileo Galilei in 1610. Ganymede, the largest of these, has a greater than that of the planet Mercury. Jupiter has been explored on several occasions by robotic spacecraft, most notably during the early Pioneer and Voyager flyby missions and later by the Galileo orbiter. In late February 2007, Jupiter was visited by the New Horizons probe, the latest probe to visit the planet is Juno, which entered into orbit around Jupiter on July 4,2016. Future targets for exploration in the Jupiter system include the probable ice-covered liquid ocean of its moon Europa, Earth and its neighbor planets may have formed from fragments of planets after collisions with Jupiter destroyed those super-Earths near the Sun. Astronomers have discovered nearly 500 planetary systems with multiple planets, Jupiter moving out of the inner Solar System would have allowed the formation of inner planets, including Earth. Jupiter is composed primarily of gaseous and liquid matter and it is the largest of the four giant planets in the Solar System and hence its largest planet. It has a diameter of 142,984 km at its equator, the average density of Jupiter,1.326 g/cm3, is the second highest of the giant planets, but lower than those of the four terrestrial planets. Jupiters upper atmosphere is about 88–92% hydrogen and 8–12% helium by percent volume of gas molecules, a helium atom has about four times as much mass as a hydrogen atom, so the composition changes when described as the proportion of mass contributed by different atoms. Thus, Jupiters atmosphere is approximately 75% hydrogen and 24% helium by mass, the atmosphere contains trace amounts of methane, water vapor, ammonia, and silicon-based compounds. There are also traces of carbon, ethane, hydrogen sulfide, neon, oxygen, phosphine, the outermost layer of the atmosphere contains crystals of frozen ammonia. The interior contains denser materials - by mass it is roughly 71% hydrogen, 24% helium, through infrared and ultraviolet measurements, trace amounts of benzene and other hydrocarbons have also been found
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Saturn
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Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with a radius about nine times that of Earth. Although it has only one-eighth the average density of Earth, with its larger volume Saturn is just over 95 times more massive, Saturn is named after the Roman god of agriculture, its astronomical symbol represents the gods sickle. Saturns interior is composed of a core of iron–nickel and rock. This core is surrounded by a layer of metallic hydrogen, an intermediate layer of liquid hydrogen and liquid helium. Saturn has a yellow hue due to ammonia crystals in its upper atmosphere. Saturns magnetic field strength is around one-twentieth of Jupiters, the outer atmosphere is generally bland and lacking in contrast, although long-lived features can appear. Wind speeds on Saturn can reach 1,800 km/h, higher than on Jupiter, sixty-two moons are known to orbit Saturn, of which fifty-three are officially named. This does not include the hundreds of moonlets comprising the rings, Saturn is a gas giant because it is predominantly composed of hydrogen and helium. It lacks a definite surface, though it may have a solid core, Saturns rotation causes it to have the shape of an oblate spheroid, that is, it is flattened at the poles and bulges at its equator. Its equatorial and polar radii differ by almost 10%,60,268 km versus 54,364 km, Jupiter, Uranus, and Neptune, the other giant planets in the Solar System, are also oblate but to a lesser extent. Saturn is the planet of the Solar System that is less dense than water—about 30% less. Although Saturns core is considerably denser than water, the specific density of the planet is 0.69 g/cm3 due to the atmosphere. Jupiter has 318 times the Earths mass, while Saturn is 95 times the mass of the Earth, together, Jupiter and Saturn hold 92% of the total planetary mass in the Solar System. On 8 January 2015, NASA reported determining the center of the planet Saturn, the temperature, pressure, and density inside Saturn all rise steadily toward the core, which causes hydrogen to transition into a metal in the deeper layers. Standard planetary models suggest that the interior of Saturn is similar to that of Jupiter, having a rocky core surrounded by hydrogen. This core is similar in composition to the Earth, but more dense, in 2004, they estimated that the core must be 9–22 times the mass of the Earth, which corresponds to a diameter of about 25,000 km. This is surrounded by a liquid metallic hydrogen layer, followed by a liquid layer of helium-saturated molecular hydrogen that gradually transitions to a gas with increasing altitude
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Planets in astrology
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Planets in astrology have a meaning different from the modern astronomical understanding of what a planet is. To the Greeks and the other earliest astronomers, this comprised the five planets visible to the naked eye. To ancient astrologers, the planets represented the will of the gods, to modern astrologers the planets represent basic drives or urges in the unconscious, or energy flow regulators representing dimensions of experience. They express themselves with different qualities in the signs of the zodiac. The planets are also related to other in the form of aspects. Modern astrologers differ on the source of the planets influence, hone writes that the planets exert it directly through gravitation or another, unknown influence. Others hold that the planets have no influence in themselves. In other words, the patterns of the universe repeat themselves everywhere, in fractal-like fashion. Therefore, the patterns that the make in the sky reflect the ebb. The planets are associated, especially in the Chinese tradition. Listed below are the meanings and domains associated with the astrological planets since ancient times. The planets in Hindu astrology are known as the Navagraha or nine realms, in Chinese astrology, the planets are associated with the life forces of yin and yang and the five elements, which play an important role in the Chinese form of geomancy known as Feng Shui. Astrologers differ on the associated with each planets exaltation. This table shows the astrological planets and the Greek and Roman deities associated with them, in most cases, the English name for planets derives from the name of a Roman god or goddess. Also of interest is the conflation of the Roman god with a similar Greek god, in some cases, it is the same deity with two different names. Treatises on the Ptolemaic planets and their influence on people born under their reign appear in book form. The seven classical planets are those seen with the naked eye. They are the Sun, Moon, Mercury, Venus, Mars, Jupiter, sometimes, the Sun and Moon were referred to as the lights or the luminaries
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Alexander von Humboldt
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Friedrich Wilhelm Heinrich Alexander von Humboldt was a Prussian geographer, naturalist, explorer, and influential proponent of Romantic philosophy and science. He was the brother of the Prussian minister, philosopher. Humboldts quantitative work on botanical geography laid the foundation for the field of biogeography, Humboldts advocacy of long-term systematic geophysical measurement laid the foundation for modern geomagnetic and meteorological monitoring. Between 1799 and 1804, Humboldt travelled extensively in Latin America and his description of the journey was written up and published in an enormous set of volumes over 21 years. Humboldt was one of the first people to propose that the lands bordering the Atlantic Ocean were once joined and this important work also motivated a holistic perception of the universe as one interacting entity. Alexander von Humboldt was born in Berlin in Prussia on 14 September 1769 and he was baptized as a baby in the Lutheran faith, with the Duke of Brunswick serving as godfather. At age 42, Alexander Georg was rewarded for his services in the Seven Years War with the post of Royal Chamberlain and he profited from the contract to lease state lotteries and tobacco sales. He first married the daughter of Prussian General Adjutant Schweder, in 1766, Alexander Georg married Maria Elisabeth Colomb, a well-educated woman and widow of Baron Hollwede, with whom she had a son. Alexander Georg and Maria Elisabeth had three children, a daughter, who died young, and then two sons, Wilhelm and Alexander and her first-born son, Wilhelms and Alexanders half-brother, was something of a neer do well, not often mentioned in the family history. Alexander Georg died in 1779, leaving the brothers Humboldt in the care of their emotionally distant mother, Humboldts mother expected them to become civil servants of the Prussian state. The money Baron Holwede left to Alexanders mother became, after her death, instrumental in funding Alexanders explorations, due to his youthful penchant for collecting and labeling plants, shells and insects, Alexander received the playful title of the little apothecary. On April 25,1789, he matriculated at Göttingen, then known for the lectures of C. G. Heyne and his brother Wilhelm was already a student at Göttingen, but they did not interact much since their intellectual interests were quite different. His vast and varied interests were by this time fully developed, at Göttingen he met Georg Forster, a naturalist who had been with Captain James Cook on his second voyage. Humboldt traveled with Forster in Europe, the two traveled to England, Humboldts first sea voyage, The Netherlands, and France. In England, he met Sir Joseph Banks, President of the Royal Society, the scientific friendship between Banks and Humboldt lasted until Bankss death in 1820, and the two shared botanical specimens for study. Banks also mobilized his scientific contacts in later years to aid Humboldts work, Humboldts scientific excursion up the Rhine resulted in his 1790 treatise Mineralogische Beobachtungen über einige Basalte am Rhein. Humboldts passion for travel was of long standing, Humboldts talents were devoted to the purpose of preparing himself as a scientific explorer. During this period, his brother Wilhelm married, but Alexander did not attend the nuptials, Humboldt graduated from the Freiberg School of Mines in 1792 and was appointed to a Prussian government position in the Department of Mines as an inspector in Bayreuth and the Fichtel mountains
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Timaeus (dialogue)
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Timaeus is one of Platos dialogues, mostly in the form of a long monologue given by the title character Timaeus of Locri, written c.360 BC. The work puts forward speculation on the nature of the physical world, participants in the dialogue include Socrates, Timaeus, Hermocrates, and Critias. Some scholars believe that it is not the Critias of the Thirty Tyrants who is appearing in this dialogue, but his grandfather and it has been suggested that Timaeus influenced a book about Pythagoras, written by Philolaus. The dialogue takes place the day after Socrates described his ideal state, in Platos works such a discussion occurs in the Republic. Hermocrates wishes to oblige Socrates and mentions that Critias knows just the account to do so, Critias proceeds to tell the story of Solons journey to Egypt where he hears the story of Atlantis, and how Athens used to be an ideal state that subsequently waged war against Atlantis. Critias believes that he is getting ahead of himself, and mentions that Timaeus will tell part of the account from the origin of the universe to man, the history of Atlantis is postponed to Critias. The main content of the dialogue, the exposition by Timaeus, Timaeus begins with a distinction between the physical world, and the eternal world. The physical one is the world changes and perishes, therefore it is the object of opinion. The eternal one never changes, therefore it is apprehended by reason, the speeches about the two worlds are conditioned by the different nature of their objects. Indeed, a description of what is changeless, fixed and clearly intelligible will be changeless and fixed, while a description of changes and is likely, will also change. As being is to becoming, so is truth to belief, therefore, in a description of the physical world, one should not look for anything more than a likely story. Timaeus suggests that since nothing becomes or changes without cause, then the cause of the universe must be a demiurge or a god, and since the universe is fair, the demiurge must have looked to the eternal model to make it, and not to the perishable one. Hence, using the eternal and perfect world of forms or ideals as a template, he set about creating our world, Timaeus continues with an explanation of the creation of the universe, which he ascribes to the handiwork of a divine craftsman. The demiurge, being good, wanted there to be as good as was the world. The demiurge is said to bring out of substance by imitating an unchanging. The ananke, often translated as necessity, was the only other co-existent element or presence in Platos cosmogony, later Platonists clarified that the eternal model existed in the mind of the Demiurge. Timaeus describes the substance as a lack of homogeneity or balance, in which the four elements were shapeless, mixed, considering that order is favourable over disorder, the essential act of the creator was to bring order and clarity to this substance. Therefore, all the properties of the world are to be explained by the choice of what is fair and good, or
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Aristotle
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Aristotle was an ancient Greek philosopher and scientist born in the city of Stagira, Chalkidice, on the northern periphery of Classical Greece. His father, Nicomachus, died when Aristotle was a child, at seventeen or eighteen years of age, he joined Platos Academy in Athens and remained there until the age of thirty-seven. Shortly after Plato died, Aristotle left Athens and, at the request of Philip II of Macedon, teaching Alexander the Great gave Aristotle many opportunities and an abundance of supplies. He established a library in the Lyceum which aided in the production of many of his hundreds of books and he believed all peoples concepts and all of their knowledge was ultimately based on perception. Aristotles views on natural sciences represent the groundwork underlying many of his works, Aristotles views on physical science profoundly shaped medieval scholarship. Their influence extended from Late Antiquity and the Early Middle Ages into the Renaissance, some of Aristotles zoological observations, such as on the hectocotyl arm of the octopus, were not confirmed or refuted until the 19th century. His works contain the earliest known study of logic, which was incorporated in the late 19th century into modern formal logic. Aristotle was well known among medieval Muslim intellectuals and revered as The First Teacher and his ethics, though always influential, gained renewed interest with the modern advent of virtue ethics. All aspects of Aristotles philosophy continue to be the object of academic study today. Though Aristotle wrote many elegant treatises and dialogues – Cicero described his style as a river of gold – it is thought that only around a third of his original output has survived. Aristotle, whose means the best purpose, was born in 384 BC in Stagira, Chalcidice. His father Nicomachus was the physician to King Amyntas of Macedon. Aristotle was orphaned at a young age, although there is little information on Aristotles childhood, he probably spent some time within the Macedonian palace, making his first connections with the Macedonian monarchy. At the age of seventeen or eighteen, Aristotle moved to Athens to continue his education at Platos Academy and he remained there for nearly twenty years before leaving Athens in 348/47 BC. Aristotle then accompanied Xenocrates to the court of his friend Hermias of Atarneus in Asia Minor, there, he traveled with Theophrastus to the island of Lesbos, where together they researched the botany and zoology of the island. Aristotle married Pythias, either Hermiass adoptive daughter or niece and she bore him a daughter, whom they also named Pythias. Soon after Hermias death, Aristotle was invited by Philip II of Macedon to become the tutor to his son Alexander in 343 BC, Aristotle was appointed as the head of the royal academy of Macedon. During that time he gave not only to Alexander
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On the Heavens
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On the Heavens is Aristotles chief cosmological treatise, written in 350 BC it contains his astronomical theory and his ideas on the concrete workings of the terrestrial world. It should not be confused with the spurious work On the Universe, according to Aristotle in On the Heavens, the heavenly bodies are the most perfect realities, whose motions are ruled by principles other than those of bodies in the sublunary sphere. Hence their motions are eternal and perfect, and the motion is the circular one. As substances, celestial bodies have matter and form, sometimes Aristotle seems to regard them as living beings with a rational soul as their form. This work is significant as one of the pillars of the Aristotelian worldview. Similarly, this work and others by Aristotle were important seminal works by which much of scholasticism was derived. Aristotelian philosophy and cosmology was influential in the Islamic world, where his ideas were taken up by the Falsafa school of philosophy throughout the half of the first millennia AD. Of these, philosophers Averroes and Avicenna are especially notable, european philosophers had a similarly complex relationship with De Caelo, attempting to reconcile church doctrine with the mathematics of Ptolemy and the structure of Aristotle. A particularly cogent example of this is in the work of Thomas Aquinas, theologian, known today as St. Otherwise, Aquinas accepted Aristotles explanation of the physical world, including his cosmology and physics. Livre du ciel et du monde was written at the command of King Charles V, stuart Leggatt, On the Heavens I and II. ISBN 0-85668-663-8 William Keith Chambers Guthrie, Aristotle On the Heavens, john Leofric Stocks, On the Heavens. Adelaide Etexts Sacred Texts InfoMotions MIT Internet Archive Free Audiobook Thomas Taylor, The treatises of Aristotle, on the heavens, on generation & corruption, ISBN 1-898910-24-3 Dalimier, C. and Pellegrin, P. Aristote. Gigon, O. Vom Himmel, Von der Seele, Von der Dichtkunst, aristoteles’ Vier Bücher über das Himmelsgebäude und Zwei Bücher über Entstehen und Vergehen. Prantl, C. De coelo, et de generatione et corruptione, physics Aristotelian physics Dynamics of the celestial spheres Celestial spheres Elders, L. Aristotle’s Cosmology, A Commentary on the De Caelo. Free audiobook version of On the Heavens
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Aratus
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Aratus was a Greek didactic poet. His major extant work is his hexameter poem Phenomena, the first half of which is a setting of a lost work of the same name by Eudoxus of Cnidus. It describes the constellations and other celestial phenomena, the second half is called the Diosemeia, and is chiefly about weather lore. There are several accounts of his life by anonymous Greek writers, from these it appears that he was a native of Soli in Cilicia. He is known to have studied with Menecrates in Ephesus and Philitas in Cos, as a disciple of the Peripatetic philosopher Praxiphanes, in Athens, he met the Stoic philosopher Zeno, as well as Callimachus of Cyrene and Menedemus, the founder of the Eretrian school. About 276 BC he was invited to the court of the Macedonian king Antigonus II Gonatas, here he wrote his most famous poem, Phenomena. He then spent some time at the court of Antiochus I Soter of Syria, but subsequently returned to Pella in Macedon and his chief pursuits were medicine, grammar, and philosophy. Several poetical works on subjects, as well as a number of prose epistles, are attributed to Aratus. These have generally been joined together as if parts of the work, but they seem to be distinct poems, the first, called Phenomena, consists of 732 verses. The Phenomena appears to be based on two prose works—Phenomena and Enoptron —by Eudoxus of Cnidus, written about a century earlier, the opening of the poem asserts the dependence of all things upon Zeus. From the lack of precision in the descriptions, it would seem that Aratus was neither a mathematician nor observer or, at any rate, that in this work he did not aim at scientific accuracy. These errors are partly to be attributed to Eudoxus himself, the Diosemeia consists of forecasts of the weather from astronomical phenomena, with an account of its effects upon animals. It appears to be an imitation of Hesiod, and to have been imitated by Virgil in some parts of the Georgics, the materials are said to be taken almost wholly from Aristotles Meteorologica, from the work of Theophrastus, On Weather Signs, and from Hesiod. Nothing is said in either poem about Hellenistic astrology, the two poems were very popular both in the Greek and Roman world, as is proved by the number of commentaries and Latin translations. He enjoyed immense prestige among Hellenistic poets, including Theocritus, Callimachus and this assessment was picked up by Latin poets, including Ovid and Virgil. Latin versions were made by other than Cicero, Ovid, the member of the imperial Julio-Claudian dynasty Germanicus. Aratus was also cited by the author of Acts, in 17.28, paul, speaking of God, quotes the fifth line of Aratuss Phenomena, Authors of twenty-seven commentaries are known, ones by Theon of Alexandria, Achilles Tatius and Hipparchus of Nicaea survive. An Arabic translation was commissioned in the century by the Caliph Al-Mamun
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Almagest
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The Almagest is the critical source of information on ancient Greek astronomy. It has also been valuable to students of mathematics because it documents the ancient Greek mathematician Hipparchuss work, Hipparchus wrote about trigonometry, but because his works appear to have been lost, mathematicians use Ptolemys book as their source for Hipparchuss work and ancient Greek trigonometry in general. The treatise was later titled Hē Megalē Syntaxis, and the form of this lies behind the Arabic name al-majisṭī. Ptolemy set up a public inscription at Canopus, Egypt, in 147 or 148, the late N. T. Hamilton found that the version of Ptolemys models set out in the Canopic Inscription was earlier than the version in the Almagest. Hence it cannot have been completed before about 150, a century after Ptolemy began observing. The Syntaxis Mathematica consists of thirteen sections, called books, an example illustrating how the Syntaxis was organized is given below. It is a 152-page Latin edition printed in 1515 at Venice by Petrus Lichtenstein, then follows an explanation of chords with table of chords, observations of the obliquity of the ecliptic, and an introduction to spherical trigonometry. There is also a study of the angles made by the ecliptic with the vertical, Book III covers the length of the year, and the motion of the Sun. Ptolemy explains Hipparchus discovery of the precession of the equinoxes and begins explaining the theory of epicycles. Books IV and V cover the motion of the Moon, lunar parallax, the motion of the apogee. Book VI covers solar and lunar eclipses, books VII and VIII cover the motions of the fixed stars, including precession of the equinoxes. They also contain a catalogue of 1022 stars, described by their positions in the constellations. The brightest stars were marked first magnitude, while the faintest visible to the eye were sixth magnitude. Each numerical magnitude was twice the brightness of the following one and this system is believed to have originated with Hipparchus. The stellar positions too are of Hipparchan origin, despite Ptolemys claim to the contrary, Book IX addresses general issues associated with creating models for the five naked eye planets, and the motion of Mercury. Book X covers the motions of Venus and Mars, Book XI covers the motions of Jupiter and Saturn. Book XII covers stations and retrograde motion, which occurs when planets appear to pause, Ptolemy understood these terms to apply to Mercury and Venus as well as the outer planets. Book XIII covers motion in latitude, that is, the deviation of planets from the ecliptic, the cosmology of the Syntaxis includes five main points, each of which is the subject of a chapter in Book I
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Ptolemy
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Claudius Ptolemy was a Greek writer, known as a mathematician, astronomer, geographer, astrologer, and poet of a single epigram in the Greek Anthology. He lived in the city of Alexandria in the Roman province of Egypt, wrote in Koine Greek, beyond that, few reliable details of his life are known. His birthplace has been given as Ptolemais Hermiou in the Thebaid in a statement by the 14th-century astronomer Theodore Meliteniotes. This is a very late attestation, however, and there is no reason to suppose that he ever lived elsewhere than Alexandria. Ptolemy wrote several treatises, three of which were of importance to later Byzantine, Islamic and European science. The first is the astronomical treatise now known as the Almagest, although it was entitled the Mathematical Treatise. The second is the Geography, which is a discussion of the geographic knowledge of the Greco-Roman world. The third is the treatise in which he attempted to adapt horoscopic astrology to the Aristotelian natural philosophy of his day. This is sometimes known as the Apotelesmatika but more known as the Tetrabiblos from the Greek meaning Four Books or by the Latin Quadripartitum. The name Claudius is a Roman nomen, the fact that Ptolemy bore it indicates he lived under the Roman rule of Egypt with the privileges and political rights of Roman citizenship. It would have suited custom if the first of Ptolemys family to become a citizen took the nomen from a Roman called Claudius who was responsible for granting citizenship, if, as was common, this was the emperor, citizenship would have been granted between AD41 and 68. The astronomer would also have had a praenomen, which remains unknown and it occurs once in Greek mythology, and is of Homeric form. All the kings after him, until Egypt became a Roman province in 30 BC, were also Ptolemies, abu Mashar recorded a belief that a different member of this royal line composed the book on astrology and attributed it to Ptolemy. The correct answer is not known”, Ptolemy wrote in Greek and can be shown to have utilized Babylonian astronomical data. He was a Roman citizen, but most scholars conclude that Ptolemy was ethnically Greek and he was often known in later Arabic sources as the Upper Egyptian, suggesting he may have had origins in southern Egypt. Later Arabic astronomers, geographers and physicists referred to him by his name in Arabic, Ptolemys Almagest is the only surviving comprehensive ancient treatise on astronomy. Ptolemy presented his models in convenient tables, which could be used to compute the future or past position of the planets. The Almagest also contains a catalogue, which is a version of a catalogue created by Hipparchus
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. Bibcode:2006AJ....132.2513S. doi:10.1086/508861. Retrieved 2015-07-15.
