1.
La Silla Observatory
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La Silla Observatory is an astronomical observatory in Chile with three telescopes built and operated by the European Southern Observatory. Several other telescopes are located at the site and are maintained by ESO. The observatory is one of the largest in the Southern Hemisphere and was the first in Chile to be used by ESO. The La Silla telescopes and instruments are located 150 km northeast of La Serena at the outskirts of the Chilean Atacama Desert, following the decision in 1963 to approve Chile as the site for the ESO observatory, scouting parties were sent to various locations to assess their suitability. The site that was decided upon was La Silla in the part of the Atacama desert,600 km north of Santiago de Chile. Besides being government property, it had the benefits of being in a dry, flat and easily accessible area, yet isolated and remote from any artificial light. Originally named the Cinchado, it was renamed La Silla after its saddle-like shape, on October 30,1964, the contracts were signed and an area of 245 square miles was purchased the following year. During 1965, temporary facilities were erected with living quarters, a workshop, the dedication ceremony of the road to the top took place in March 1966, two months after its completion. On 25 March 1969, the ESO site at La Silla was finally inaugurated by President Eduardo Frei Montalva. With a permanent base of dormitories, workshops, hotels and several functioning telescopes, the observatory was fully operational. The ESO1. 5-metre and ESO 1-metre telescopes had been erected in the late 1960s and these three telescopes can be seen in this order from right to left in the background of the adjunct image from June 1968. By 1976, the largest telescope planned, the § ESO3.6 m Telescope and it was subsequently to have a 1. 4m CAT attached. In 1984, the 2. 2m telescope began operations, while in March 1989, a 1-metre telescope owned by Marseille Observatory opened in 1998, followed by a 1. 2-metre telescope from Geneva Observatory in 2000. ESO operates three major optical and near infrared telescopes at the La Silla site, the New Technology Telescope, the 3. 6-m ESO Telescope, and these telescopes are not operated by ESO and hence do not fall under the responsibility of La Silla Science Operations. The telescope hosts HARPS, the High Accuracy Radial velocity Planet Searcher, HARPS is a spectrograph with unrivalled precision and is the most successful finder of low-mass exoplanets to date. Since April 2008, HARPS is the instrument available at the 3.6 m telescope. The ESO New Technology Telescope is an Alt-Az,3. 58-metre Richey-Chretien telescope which pioneered the use of active optics, the telescope and its enclosure had a revolutionary design for optimal image quality. NTT saw first light in March 1989, the telescope chamber is ventilated by a system of flaps which optimize the air flow across the NTT minimizing the dome and mirror seeing
2.
Astronomy
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Astronomy is a natural science that studies celestial objects and phenomena. It applies mathematics, physics, and chemistry, in an effort to explain the origin of those objects and phenomena and their evolution. Objects of interest include planets, moons, stars, galaxies, and comets, while the phenomena include supernovae explosions, gamma ray bursts, more generally, all astronomical phenomena that originate outside Earths atmosphere are within the purview of astronomy. A related but distinct subject, physical cosmology, is concerned with the study of the Universe as a whole, Astronomy is the oldest of the natural sciences. The early civilizations in recorded history, such as the Babylonians, Greeks, Indians, Egyptians, Nubians, Iranians, Chinese, during the 20th century, the field of professional astronomy split into observational and theoretical branches. Observational astronomy is focused on acquiring data from observations of astronomical objects, theoretical astronomy is oriented toward the development of computer or analytical models to describe astronomical objects and phenomena. The two fields complement each other, with theoretical astronomy seeking to explain the results and observations being used to confirm theoretical results. Astronomy is one of the few sciences where amateurs can play an active role, especially in the discovery. Amateur astronomers have made and contributed to many important astronomical discoveries, Astronomy means law of the stars. Astronomy should not be confused with astrology, the system which claims that human affairs are correlated with the positions of celestial objects. Although the two share a common origin, they are now entirely distinct. Generally, either the term astronomy or astrophysics may be used to refer to this subject, however, since most modern astronomical research deals with subjects related to physics, modern astronomy could actually be called astrophysics. Few fields, such as astrometry, are purely astronomy rather than also astrophysics, some titles of the leading scientific journals in this field includeThe Astronomical Journal, The Astrophysical Journal and Astronomy and Astrophysics. In early times, astronomy only comprised the observation and predictions of the motions of objects visible to the naked eye, in some locations, early cultures assembled massive artifacts that possibly had some astronomical purpose. Before tools such as the telescope were invented, early study of the stars was conducted using the naked eye, most of early astronomy actually consisted of mapping the positions of the stars and planets, a science now referred to as astrometry. From these observations, early ideas about the motions of the planets were formed, and the nature of the Sun, Moon, the Earth was believed to be the center of the Universe with the Sun, the Moon and the stars rotating around it. This is known as the model of the Universe, or the Ptolemaic system. The Babylonians discovered that lunar eclipses recurred in a cycle known as a saros
3.
Ancient Greek
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Ancient Greek includes the forms of Greek used in ancient Greece and the ancient world from around the 9th century BC to the 6th century AD. It is often divided into the Archaic period, Classical period. It is antedated in the second millennium BC by Mycenaean Greek, the language of the Hellenistic phase is known as Koine. Koine is regarded as a historical stage of its own, although in its earliest form it closely resembled Attic Greek. Prior to the Koine period, Greek of the classic and earlier periods included several regional dialects, Ancient Greek was the language of Homer and of fifth-century Athenian historians, playwrights, and philosophers. It has contributed many words to English vocabulary and has been a subject of study in educational institutions of the Western world since the Renaissance. This article primarily contains information about the Epic and Classical phases of the language, Ancient Greek was a pluricentric language, divided into many dialects. The main dialect groups are Attic and Ionic, Aeolic, Arcadocypriot, some dialects are found in standardized literary forms used in literature, while others are attested only in inscriptions. There are also several historical forms, homeric Greek is a literary form of Archaic Greek used in the epic poems, the Iliad and Odyssey, and in later poems by other authors. Homeric Greek had significant differences in grammar and pronunciation from Classical Attic, the origins, early form and development of the Hellenic language family are not well understood because of a lack of contemporaneous evidence. Several theories exist about what Hellenic dialect groups may have existed between the divergence of early Greek-like speech from the common Proto-Indo-European language and the Classical period and they have the same general outline, but differ in some of the detail. The invasion would not be Dorian unless the invaders had some relationship to the historical Dorians. The invasion is known to have displaced population to the later Attic-Ionic regions, the Greeks of this period believed there were three major divisions of all Greek people—Dorians, Aeolians, and Ionians, each with their own defining and distinctive dialects. Often non-west is called East Greek, Arcadocypriot apparently descended more closely from the Mycenaean Greek of the Bronze Age. Boeotian had come under a strong Northwest Greek influence, and can in some respects be considered a transitional dialect, thessalian likewise had come under Northwest Greek influence, though to a lesser degree. Most of the dialect sub-groups listed above had further subdivisions, generally equivalent to a city-state and its surrounding territory, Doric notably had several intermediate divisions as well, into Island Doric, Southern Peloponnesus Doric, and Northern Peloponnesus Doric. The Lesbian dialect was Aeolic Greek and this dialect slowly replaced most of the older dialects, although Doric dialect has survived in the Tsakonian language, which is spoken in the region of modern Sparta. Doric has also passed down its aorist terminations into most verbs of Demotic Greek, by about the 6th century AD, the Koine had slowly metamorphosized into Medieval Greek
4.
Celestial bodies
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An astronomical object or celestial object is a naturally occurring physical entity, association, or structure that current astronomy has demonstrated to exist in the observable universe. In astronomy, the object and body are often used interchangeably. Examples for astronomical objects include planetary systems, star clusters, nebulae and galaxies, while asteroids, moons, planets, and stars are astronomical bodies. A comet may be identified as both body and object, It is a body when referring to the nucleus of ice and dust. The universe can be viewed as having a hierarchical structure, at the largest scales, the fundamental component of assembly is the galaxy. Galaxies are organized groups and clusters, often within larger superclusters. Disc galaxies encompass lenticular and spiral galaxies with features, such as spiral arms, at the core, most galaxies have a supermassive black hole, which may result in an active galactic nucleus. Galaxies can also have satellites in the form of dwarf galaxies, the constituents of a galaxy are formed out of gaseous matter that assembles through gravitational self-attraction in a hierarchical manner. At this level, the fundamental components are the stars. The great variety of forms are determined almost entirely by the mass, composition. Stars may be found in systems that orbit about each other in a hierarchical organization. A planetary system and various objects such as asteroids, comets and debris. The various distinctive types of stars are shown by the Hertzsprung–Russell diagram —a plot of stellar luminosity versus surface temperature. Each star follows a track across this diagram. If this track takes the star through a region containing a variable type. An example of this is the instability strip, a region of the H-R diagram that includes Delta Scuti, RR Lyrae, the table below lists the general categories of bodies and objects by their location or structure. International Astronomical Naming Commission List of light sources List of Solar System objects Lists of astronomical objects SkyChart, Sky & Telescope Monthly skymaps for every location on Earth
5.
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
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.
Curiosity (rover)
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Curiosity is a car-sized robotic rover exploring Gale Crater on Mars as part of NASAs Mars Science Laboratory mission. Curiosity was launched from Cape Canaveral on November 26,2011, at 15,02 UTC aboard the MSL spacecraft, the Bradbury Landing site was less than 2.4 km from the center of the rovers touchdown target after a 560 million km journey. Curiositys design will serve as the basis for the planned Mars 2020 rover, in December 2012, Curiositys two-year mission was extended indefinitely. As of April 7,2017, Curiosity has been on Mars for 1659 sols since landing on August 6,2012, the mission will also help prepare for human exploration. This data would be important for a manned mission. The remaining mass of the MSL craft was discarded in the process of carrying out this task, dimensions, Curiosity has a mass of 899 kg including 80 kg of scientific instruments. The rover is 2.9 m long by 2.7 m wide by 2.2 m in height, power source, Curiosity is powered by a radioisotope thermoelectric generator, like the successful Viking 1 and Viking 2 Mars landers in 1976. Radioisotope power systems are generators that produce electricity from the decay of isotopes, such as plutonium-238. Heat given off by the decay of this isotope is converted into electric voltage by thermocouples, providing constant power during all seasons and through the day, waste heat can be used via pipes to warm systems, freeing electrical power for the operation of the vehicle and instruments. Curiositys RTG is fueled by 4.8 kg of plutonium-238 dioxide supplied by the U. S. Department of Energy. The MMRTG produces less power over time as its plutonium fuel decays, at its minimum lifetime of 14 years, electrical power output is down to 100 watts. The power source will generate 9 MJ each day, much more than the solar panels of the Mars Exploration Rovers, the electrical output from the MMRTG charges two rechargeable lithium-ion batteries. This enables the power subsystem to meet power demands of rover activities when the demand temporarily exceeds the generator’s steady output level. Each battery has a capacity of about 42 ampere-hours, heat rejection system, The temperatures at the landing site can vary from −127 to 40 °C, therefore, the thermal system will warm the rover for most of the Martian year. It uses fluid pumped through 60 m of tubing in the body so that sensitive components are kept at optimal temperatures. The HRS also has the ability to cool components if necessary, the computers run the VxWorks real-time operating system. Each computers memory includes 256 kB of EEPROM,256 MB of DRAM, for comparison, the Mars Exploration Rovers used 3 MB of EEPROM,128 MB of DRAM, and 256 MB of flash memory. The RCE computers use the RAD750 CPU, which is a successor to the RAD6000 CPU of the Mars Exploration Rovers
8.
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
9.
Earth
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Earth, otherwise known as the World, or the Globe, is the third planet from the Sun and the only object in the Universe known to harbor life. It is the densest planet in the Solar System and the largest of the four terrestrial planets, according to radiometric dating and other sources of evidence, Earth formed about 4.54 billion years ago. Earths gravity interacts with objects in space, especially the Sun. During one orbit around the Sun, Earth rotates about its axis over 365 times, thus, Earths axis of rotation is tilted, producing seasonal variations on the planets surface. The gravitational interaction between the Earth and Moon causes ocean tides, stabilizes the Earths orientation on its axis, Earths lithosphere is divided into several rigid tectonic plates that migrate across the surface over periods of many millions of years. About 71% of Earths surface is covered with water, mostly by its oceans, the remaining 29% is land consisting of continents and islands that together have many lakes, rivers and other sources of water that contribute to the hydrosphere. The majority of Earths polar regions are covered in ice, including the Antarctic ice sheet, Earths interior remains active with a solid iron inner core, a liquid outer core that generates the Earths magnetic field, and a convecting mantle that drives plate tectonics. Within the first billion years of Earths history, life appeared in the oceans and began to affect the Earths atmosphere and surface, some geological evidence indicates that life may have arisen as much as 4.1 billion years ago. Since then, the combination of Earths distance from the Sun, physical properties, in the history of the Earth, biodiversity has gone through long periods of expansion, occasionally punctuated by mass extinction events. Over 99% of all species that lived on Earth are extinct. Estimates of the number of species on Earth today vary widely, over 7.4 billion humans live on Earth and depend on its biosphere and minerals for their survival. Humans have developed diverse societies and cultures, politically, the world has about 200 sovereign states, the modern English word Earth developed from a wide variety of Middle English forms, which derived from an Old English noun most often spelled eorðe. It has cognates in every Germanic language, and their proto-Germanic root has been reconstructed as *erþō, originally, earth was written in lowercase, and from early Middle English, its definite sense as the globe was expressed as the earth. By early Modern English, many nouns were capitalized, and the became the Earth. More recently, the name is simply given as Earth. House styles now vary, Oxford spelling recognizes the lowercase form as the most common, another convention capitalizes Earth when appearing as a name but writes it in lowercase when preceded by the. It almost always appears in lowercase in colloquial expressions such as what on earth are you doing, the oldest material found in the Solar System is dated to 4. 5672±0.0006 billion years ago. By 4. 54±0.04 Gya the primordial Earth had formed, the formation and evolution of Solar System bodies occurred along with the Sun
10.
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
11.
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
12.
Conjunction (astronomy)
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In astronomy, a conjunction occurs when two astronomical objects or spacecraft have either the same right ascension or the same ecliptic longitude, usually as observed from Earth. The astronomical symbol for conjunction is ☌ and handwritten, the conjunction symbol is not used in modern astronomy. It continues to be used in astrology, a related word, appulse, is the minimum apparent separation on the sky of two astronomical objects. Conjunctions involve either two objects in the Solar System or one object in the Solar System and a distant object. A conjunction is an apparent phenomenon caused by the observers perspective, conjunctions between two bright objects close to the ecliptic, such as two bright planets, can be seen with the naked eye. More generally, in the case of two planets, it means that they merely have the same right ascension. This is called conjunction in right ascension, however, there is also the term conjunction in ecliptic longitude. At such conjunction both objects have the same ecliptic longitude, conjunction in right ascension and conjunction in ecliptic longitude do not normally take place at the same time, but in most cases nearly at the same time. However, at triple conjunctions, it is possible that a conjunction only in right ascension occurs, at the time of conjunction – it does not matter if in right ascension or in ecliptic longitude – the involved planets are close together upon the celestial sphere. In the vast majority of cases, one of the planets will appear to pass north or south of the other. However, if two celestial bodies attain the same declination at the time of a conjunction in right ascension, in such a case, a syzygy takes place. If one object moves into the shadow of another, the event is an eclipse, for example, if the Moon passes into the shadow of Earth and disappears from view, this event is called a lunar eclipse. If the visible disk of the object is considerably smaller than that of the farther object. When Mercury passes in front of the Sun, it is a transit of Mercury, when the nearer object appears larger than the farther one, it will completely obscure its smaller companion, this is called an occultation. An example of an occultation is when the Moon passes between Earth and the Sun, causing the Sun to disappear either entirely or partially and this phenomenon is commonly known as a solar eclipse. Occultations in which the body is neither the Sun nor the Moon are very rare. More frequent, however, is an occultation of a planet by the Moon, several such events are visible every year from various places on Earth. A conjunction, as a phenomenon of perspective, is an event that involves two astronomical bodies seen by an observer on the Earth
13.
Opposition (planets)
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In positional astronomy, two celestial bodies are said to be in opposition when they are on opposite sides of the sky, viewed from a given place. A planet is said to be in opposition when it is in opposition to the Sun, opposition occurs only in superior planets. The instant of opposition is defined as that when the apparent geocentric longitude of the body differs by 180° from the apparent geocentric longitude of the Sun. When it is in opposition, a lunar eclipse occurs. The astronomical symbol for opposition is ☍, handwritten, As seen from a planet that is superior, an inferior planet on the opposite side of the Sun is in superior conjunction with the Sun. An inferior conjunction occurs when the two lie in a line on the same side of the Sun. At inferior conjunction, the planet is in opposition to the Sun as seen from the inferior planet. Conjunction Positional astronomy Syzygy Asteroids around opposition – British Astronomical Association - Computing Section
14.
Solar eclipse
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As seen from the Earth, a solar eclipse is a type of eclipse that occurs when the Moon passes between the Sun and Earth, and the Moon fully or partially blocks the Sun. This can happen only at new moon when the Sun and the Moon are in conjunction as seen from Earth in an alignment referred to as syzygy, in a total eclipse, the disk of the Sun is fully obscured by the Moon. In partial and annular eclipses, only part of the Sun is obscured, if the Moon were in a perfectly circular orbit, a little closer to the Earth, and in the same orbital plane, there would be total solar eclipses every month. However, the Moons orbit is inclined at more than 5 degrees to the Earths orbit around the Sun, Earths orbit is called the ecliptic plane as the Moons orbit must cross this plane in order for an eclipse to occur. In addition, the Moons actual orbit is elliptical, often taking it far away from Earth that its apparent size is not large enough to block the Sun totally. The orbital planes cross each other at a line of nodes resulting in at least two, and up to five, solar eclipses occurring each year, no more than two of which can be total eclipses. However, total solar eclipses are rare at any particular location because totality exists only along a path on the Earths surface traced by the Moons shadow or umbra. An eclipse is a natural phenomenon, nevertheless, in some ancient and modern cultures, solar eclipses have been attributed to supernatural causes or regarded as bad omens. A total solar eclipse can be frightening to people who are unaware of its explanation, as the Sun seems to disappear during the day. People referred to as eclipse chasers or umbraphiles will travel to locations to observe or witness predicted central solar eclipses. For the date of the next eclipse see the section Recent, during any one eclipse, totality occurs at best only in a narrow track on the surface of Earth. An annular eclipse occurs when the Sun and Moon are exactly in line with the Earth, hence the Sun appears as a very bright ring, or annulus, surrounding the dark disk of the Moon. A hybrid eclipse shifts between a total and annular eclipse, at certain points on the surface of Earth, it appears as a total eclipse, whereas at other points it appears as annular. A partial eclipse occurs when the Sun and Moon are not exactly in line with the Earth and this phenomenon can usually be seen from a large part of the Earth outside of the track of an annular or total eclipse. However, some eclipses can only be seen as an eclipse, because the umbra passes above the Earths polar regions. Partial eclipses are virtually unnoticeable in terms of the suns brightness, even at 99%, it would be no darker than civil twilight. Of course, partial eclipses can be observed if one is viewing the sun through a darkening filter, the Suns distance from Earth is about 400 times the Moons distance, and the Suns diameter is about 400 times the Moons diameter. Because these ratios are approximately the same, the Sun and the Moon as seen from Earth appear to be approximately the same size, about 0.5 degree of arc in angular measure
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Lunar eclipse
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A lunar eclipse occurs when the Moon passes directly behind the Earth into its umbra. This can occur only when the sun, Earth, and moon are aligned exactly, or very closely so, hence, a lunar eclipse can occur only the night of a full moon. The type and length of an eclipse depend upon the Moons location relative to its orbital nodes, a total lunar eclipse has the direct sunlight completely blocked by the earths shadow. The only light seen is refracted through the earths shadow and this light looks red for the same reason that the sunset looks red, due to rayleigh scattering of the more blue light. Because of its color, a total lunar eclipse is sometimes called a blood moon. Unlike a solar eclipse, which can be viewed only from a relatively small area of the world. A lunar eclipse lasts for a few hours, whereas a total eclipse lasts for only a few minutes at any given place. Also unlike solar eclipses, lunar eclipses are safe to view without any eye protection or special precautions, for the date of the next eclipse see the section Recent and forthcoming lunar eclipses. The shadow of the Earth can be divided into two parts, the umbra and penumbra. Within the umbra, there is no solar radiation. However, as a result of the Suns large angular size, solar illumination is only partially blocked in the portion of the Earths shadow. A penumbral eclipse occurs when the moon passes through the Earths penumbra, the penumbra causes a subtle darkening of the moons surface. A special type of eclipse is a total penumbral eclipse. Total penumbral eclipses are rare, and when these occur, that portion of the moon which is closest to the umbra can appear darker than the rest of the moon. A partial lunar eclipse occurs when only a portion of the moon enters the umbra, when the moon travels completely into the Earths umbra, one observes a total lunar eclipse. The moons speed through the shadow is about one kilometer per second, nevertheless, the total time between the moons first and last contact with the shadow is much longer and could last up to four hours. The relative distance of the moon from the Earth at the time of an eclipse can affect the eclipses duration, in particular, when the moon is near its apogee, the farthest point from the Earth in its orbit, its orbital speed is the slowest. The diameter of the umbra does not decrease appreciably within the changes in the distance of the moon
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Eclipse
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An eclipse occurs during a syzygy. The term eclipse is most often used to either a solar eclipse, when the Moons shadow crosses the Earths surface, or a lunar eclipse. A binary star system can also produce eclipses if the plane of the orbit of its constituent stars intersects the observers position, for any two objects in space, a line can be extended from the first through the second. The latter object will block some amount of light being emitted by the former, as viewed from such a location, this shadowing event is known as an eclipse. Typically the cross-section of the involved in an astronomical eclipse are roughly disk shaped. The region of an objects shadow during an eclipse is divided into three parts, The umbra, within which the object completely covers the light source, for the Sun, this light source is the photosphere. The antumbra, extending beyond the tip of the umbra, within which the object is completely in front of the light source, the penumbra, within which the object is only partially in front of the light source. A total eclipse occurs when the observer is within the umbra, an annular eclipse when the observer is within the antumbra, during a lunar eclipse only the umbra and penumbra are applicable. This is because Earths apparent diameter from the viewpoint of the Moon is nearly four times that of the Sun, the same terms may be used analogously in describing other eclipses, e. g. the antumbra of Deimos crossing Mars, or Phobos entering Marss penumbra. For Earth, on average L is equal to 1. 384×106 km, hence the umbral cone of the Earth can completely envelop the Moon during a lunar eclipse. If the occulting object has an atmosphere, however, some of the luminosity of the star can be refracted into the volume of the umbra. This occurs, for example, during an eclipse of the Moon by the Earth—producing a faint, on Earth, the shadow cast during an eclipse moves very approximately at 1 km per sec. This depends on the location of the shadow on the Earth, http, //www. sciforums. com/threads/speed-of-eclipse-shadow. 53722/ An eclipse cycle takes place when a series of eclipses are separated by a certain interval of time. This happens when the orbital motions of the bodies form repeating harmonic patterns, a particular instance is the saros, which results in a repetition of a solar or lunar eclipse every 6,585.3 days, or a little over 18 years. Because this is not a number of days, successive eclipses will be visible from different parts of the world. An eclipse involving the Sun, Earth, and Moon can occur only when they are nearly in a line, allowing one to be hidden behind another. Because the orbital plane of the Moon is tilted with respect to the plane of the Earth. The Sun, Earth and nodes are aligned twice a year, there can be from four to seven eclipses in a calendar year, which repeat according to various eclipse cycles, such as a saros
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Transit (astronomy)
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The word transit refers to cases where the nearer object appears considerably smaller than the more distant object. Cases where the object appears larger and completely hides the more distant object are known as occultations. One example of a transit involves the motion of a planet between an observer and the Sun. This can happen only with inferior planets, namely Mercury and Venus, however, as seen from outer planets such as Mars, the Earth itself transits the Sun on occasion. The term can also be used to describe the motion of a satellite across its parent planet, for one of the Galilean satellites across Jupiter. A transit requires three bodies to be lined up in a single line, more rare are cases where four bodies are lined up. The one closest to the present occurred on 27 June 1586, when Mercury transited the Sun as seen from Venus at the time as a transit of Mercury from Saturn. In recent years the discovery of planets has excited interest in the possibility of detecting their transits across their own stellar primaries. HD 209458b is the first such transiting planet to be discovered, in rare cases, one planet can pass in front of another. If the nearer planet appears smaller than the distant one. During a transit there are four contacts, when the circumference of the circle touches the circumference of the large circle at a single point. Historically, measuring the time of each point of contact was one of the most accurate ways to determine the positions of astronomical bodies. No missions were planned to coincide with the transit of Earth visible from Mars on 11 May 1984, consequently, the next opportunity to observe such an alignment will be in 2084. On December 21,2012, the Cassini–Huygens probe, in orbit around Saturn, on 3 June 2014, the Mars rover Curiosity observed the planet Mercury transiting the Sun, marking the first time a planetary transit has been observed from a celestial body besides Earth. 1989, ISBN 0-943396-25-5 Jean Meeus, Astronomical Tables of the Sun, Moon,1995, ISBN 0-943396-45-X Karl Ramsayer, Geodätische Astronomie, Vol. 2a of Handbuch der Vermessungskunde,900 p. J. B. Metzler, Stuttgart 1969
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Occultation
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An occultation is an event that occurs when one object is hidden by another object that passes between it and the observer. The word is used in astronomy and it can also refer to any situation wherein an object in the foreground blocks from view an object in the background. In this general sense, occultation applies to the visual scene observed from low-flying aircraft wherein foreground objects obscure distant objects dynamically, as the scene changes over time. The term occultation is most frequently used to describe those relatively frequent occasions when the Moon passes in front of a star during the course of its orbital motion around the Earth. Events that take place on the Moons dark limb are of particular interest to observers, the Moons orbit is inclined to the ecliptic, and any stars with an ecliptic latitude of less than about 6.5 degrees may be occulted by it. There are three first magnitude stars that are close to the ecliptic that they may be occulted by the Moon and by planets – Regulus, Spica. Occultations of Aldebaran are presently only possible by the Moon, because the planets pass Aldebaran to the north, neither planetary nor lunar occultations of Pollux are currently possible. However, in the far future, occultations of Pollux will be possible, some deep-sky objects, such as the Pleiades, can also be occulted by the Moon. From an observational and scientific standpoint, these grazes are the most dynamic, the accurate timing of lunar occultations is performed regularly by astronomers. Lunar occultations timed to an accuracy of a few tenths of a second have various scientific uses, photoelectric analysis of lunar occultations have also discovered some stars to be very close visual or spectroscopic binaries. Some angular diameters of stars have been measured by timing of lunar occultations, several times during the year, someone on Earth can usually observe the Moon occulting a planet. Since planets, unlike stars, have significant angular sizes, lunar occultations of planets will create a zone on Earth from which a partial occultation of the planet will occur. An observer located within that narrow zone could observe the planets disk partly blocked by the moving moon. The same mechanic can be seen with the Sun, where observers on Earth will view it as a Solar Eclipse, therefore, a Total Solar Eclipse is effectively the same event as the Moon occulting the Sun. Stars may also be occulted by planets, uranuss rings were first discovered when that planet occulted a star in 1977. On 3 July 1989, Saturn passed in front of the 5th magnitude star 28 Sagittarii, pluto occulted stars in 1988,2002, and 2006, allowing its tenuous atmosphere to be studied via atmospheric limb sounding. In rare cases, one planet can pass in front of another, if the nearer planet appears larger than the more distant one, the event is called a mutual planetary occultation. An asteroid occultation occurs when an asteroid passes in front of a star, several events occur nearly every day over the world
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New moon
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In astronomy, new moon is the first phase of the Moon, when it orbits not seen from the Earth, the moment when the Moon and the Sun have the same ecliptical longitude. The Moon is not visible at this time except when it is seen in silhouette during a solar eclipse when it is illuminated by earthshine, see the article on phases of the Moon for further details. A lunation or synodic month is the time from one new moon to the next. In the J2000.0 epoch, the length of a lunation is 29.530588 days. However, the length of any one month can vary from 29.26 to 29.80 days due to the perturbing effects of the Suns gravity on the Moons eccentric orbit. In a lunar calendar, each corresponds to a lunation. Each lunar cycle can be assigned a unique Lunation Number to identify it, the length of a lunation is about 29.53 days. Its precise duration is linked to many phenomena in nature, such as the variation between spring and neap tides, an approximate formula to compute the mean moments of new moon for successive months is, d =5.597661 +29. For all new moons between 1601 and 2401, the difference is 0.592 days = 14h13m in either direction. The duration of a lunation varies in this period between 29.272 and 29.833 days, i. e. −0. 259d = 6h12m shorter, or +0. 302d = 7h15m longer than average. This range is smaller than the difference between mean and true conjunction, because during one lunation the periodic terms cannot all change to their maximum opposite value, see the article on the full moon cycle for a fairly simple method to compute the moment of new moon more accurately. These are now outdated, Chapront et al. published improved parameters, quadratic term, In ELP2000–85, D has a quadratic term of −5. 8681T2, expressed in lunations N, this yields a correction of +87. 403×10–12N2 days to the time of conjunction. The term includes a contribution of 0. 5×. The most current estimate from Lunar Laser Ranging for the acceleration is, therefore, the new quadratic term of D is = -6. 8498T2. Indeed, the polynomial provided by Chapront et alii provides the same value and this translates to a correction of +14. 622×10−12N2 days to the time of conjunction, the quadratic term now is, +102. 026×10−12N2 days. For UT, analysis of historical observations shows that ΔT has an increase of +31 s/cy2. Converted to days and lunations, the correction from ET to UT becomes, the theoretical tidal contribution to ΔT is about +42 s/cy2 the smaller observed value is thought to be mostly due to changes in the shape of the Earth. Because the discrepancy is not fully explained, uncertainty of our prediction of UT may be as large as the difference between these values,11 s/cy2
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Full moon
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A full moon is the lunar phase that occurs when the Moon is completely illuminated as seen from Earth. This occurs when Earth is located directly between the Sun and the Moon and this means that the hemisphere of the Moon that is facing Earth is almost fully illuminated by the Sun and appears round. On some occasions at the time of full moon there is also a lunar eclipse so the face appears reddish due to the rayleigh scattering of blue light in Earths atmosphere. Lunar eclipses can occur only at full moon, where the Moons orbit allows it to pass through Earths shadow, Lunar eclipses do not occur every month because the Moon usually passes above or below Earths shadow. Lunar eclipses can only when the full moon occurs near the two nodes of the orbit, either the ascending or descending node. This causes eclipses to occur about every 6 months. The time interval between similar lunar phases—the synodic month—averages about 29.53 days, therefore, in those lunar calendars in which each month begins on the new moon, the full moon falls on either the 14th or 15th of the lunar month. Because calendar months have a number of days, lunar months may be either 29 or 30 days long. A full moon is thought of as an event of a full nights duration. This is somewhat misleading because the Moon seen from Earth is continuously becoming larger and/or smaller and its absolute maximum size occurs at the moment expansion has stopped. For any given location, about half of these absolute maximum full moons will be visible, as the other half occur during the day. Many almanacs list full moons not just by date, but by their time as well. Typical monthly calendars that include phases of the Moon may be offset by one day if intended for use in a different time zone, Full moons are generally a poor time to conduct astronomical observations, because the bright reflected sunlight from the Moon overwhelms the dimmer light from stars. On 12 December 2008, the moon occurred closer to the Earth than it had been at any time for the previous 15 years. On 19 March 2011, another full moon occurred, closer to the Earth than at any time for the previous ~18 years. On 14 November 2016, a full moon occurred closer to the Earth than at any time for the previous 68 years. The true time of a full moon may differ from this approximation by up to about 14.5 hours as a result of the non-circularity of the moons orbit, see New moon for an explanation of the formula and its parameters. The age and apparent size of the full moon vary in a cycle of just under 14 synodic months, Full moons are traditionally associated with temporal insomnia, insanity and various magical phenomena such as lycanthropy
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GMT
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Greenwich Mean Time is the mean solar time at the Royal Observatory in Greenwich, London. GMT was formerly used as the civil time standard, now superseded in that function by Coordinated Universal Time. Today GMT is considered equivalent to UTC for UK civil purposes and for navigation is considered equivalent to UT1, consequently, the term GMT should not be used for precise purposes. Due to Earths uneven speed in its orbit and its axial tilt, noon GMT is rarely the exact moment the sun crosses the Greenwich meridian. This event may occur up to 16 minutes before or after noon GMT, noon GMT is the annual average moment of this event, which accounts for the word mean in Greenwich Mean Time. Originally, astronomers considered a GMT day to start at noon while for almost everyone else it started at midnight, to avoid confusion, the name Universal Time was introduced to denote GMT as counted from midnight. Astronomers preferred the old convention to simplify their observational data, so each night was logged under a single calendar date. Today Universal Time usually refers to UTC or UT1, in some countries Greenwich Mean Time is the legal time in the winter and the population uses the term. For an explanation of why this is, see GMT in legislation below, synchronisation of the chronometer on GMT did not affect shipboard time, which was still solar time. Most time zones were based upon GMT, as an offset of a number of hours ahead of GMT or behind GMT and it was gradually adopted for other purposes, but a legal case in 1858 held local mean time to be the official time. On 14 May 1880, a signed by Clerk to Justices appeared in The Times, stating that Greenwich time is now kept almost throughout England. For example, our polling booths were opened, say, at 813 and closed at 413 PM. This was changed later in 1880, GMT was adopted on the Isle of Man in 1883, Jersey in 1898 and Guernsey in 1913. Ireland adopted GMT in 1916, supplanting Dublin Mean Time, hourly time signals from Greenwich Observatory were first broadcast on 5 February 1924, rendering the time ball at the observatory redundant in the process. The daily rotation of the Earth is irregular and constantly slows, on 1 January 1972, GMT was superseded as the international civil time standard by Coordinated Universal Time, maintained by an ensemble of atomic clocks around the world. Indeed, even the Greenwich meridian itself is not quite what it used to be—defined by the centre of the instrument at the Observatory at Greenwich. Nevertheless, the line in the old observatorys courtyard today differs no more than a few metres from that line which is now the prime meridian of the world. Historically GMT has been used two different conventions for numbering hours. The long-standing astronomical convention dating from the work of Ptolemy, was to refer to noon as zero hours and this contrasted with the civil convention of referring to midnight as zero hours dating from the Romans
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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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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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Transit of Mercury from Mars
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A transit of Mercury across the Sun as seen from Mars takes place when the planet Mercury passes directly between the Sun and Mars, obscuring a small part of the Suns disc for an observer on Mars. During a transit, Mercury can be seen from Mars as a black disc moving across the face of the Sun. Transits of Mercury from Mars are roughly two more common than transits of Mercury from Earth, there are several per decade. No human has ever seen a transit of Mercury from Mars. The Mars rovers Spirit and Opportunity could have observed the transit of January 12,2005 and they were able to observe transits of Deimos across the Sun, but at 2 angular diameter, Deimos is about 20 times larger than Mercurys 6.1 angular diameter. The rover Curiosity observed the Mercury transit of June 3,2014, the Mercury-Mars synodic period is 100.888 days. It can be calculated using the formula 1/, where P is the period of Mercury. The inclination of Mercurys orbit with respect to that of Mars is 5. 16°, which is less than its value of 7. 00° with respect to Earths ecliptic. The simultaneous occurrence of a transit of Mercury and a transit of Venus is extremely rare, but somehow more frequent than from Earth, and will next occur in the years 18,713,19,536 and 20,029. On several occasions a related event is predicted, a transit of Mercury, on November 28,3867 there is a transit of Earth and Moon, and two days later a transit of Mercury occurs. On January 16,18551 transits of Mercury and Venus are separated by 14 hours, giorgini, J. D. Yeomans, D. K. Chamberlin, A. B. Chodas, P. W. Jacobson, R. A. JPLs On-Line Solar System Data Service, Bulletin of the American Astronomical Society 28,1158,1996, transits of Mercury on Mars - Fifteen millennium catalog,5000 BC -10000 AD JPL Horizons
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Satellite
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In the context of spaceflight, a satellite is an artificial object which has been intentionally placed into orbit. Such objects are called artificial satellites to distinguish them from natural satellites such as Earths Moon. In 1957 the Soviet Union launched the worlds first artificial satellite, since then, about 6,600 satellites from more than 40 countries have been launched. According to a 2013 estimate,3,600 remained in orbit, of those, about 1,000 were operational, the rest have lived out their useful lives and become space debris. Approximately 500 operational satellites are in orbit,50 are in medium-Earth orbit. A few large satellites have been launched in parts and assembled in orbit. Over a dozen space probes have been placed into orbit around other bodies and become artificial satellites to the Moon, Mercury, Venus, Mars, Jupiter, Saturn, a few asteroids, Satellites are used for many purposes. Common types include military and civilian Earth observation satellites, communications satellites, navigation satellites, weather satellites, Space stations and human spacecraft in orbit are also satellites. Satellite orbits vary greatly, depending on the purpose of the satellite, well-known classes include low Earth orbit, polar orbit, and geostationary orbit. A launch vehicle is a rocket that throws a satellite into orbit, usually it lifts off from a launch pad on land. Some are launched at sea from a submarine or a mobile maritime platform, Satellites are usually semi-independent computer-controlled systems. Satellite subsystems attend many tasks, such as power generation, thermal control, telemetry, attitude control, the first fictional depiction of a satellite being launched into orbit was a short story by Edward Everett Hale, The Brick Moon. The idea surfaced again in Jules Vernes The Begums Fortune, in 1903, Konstantin Tsiolkovsky published Exploring Space Using Jet Propulsion Devices, which is the first academic treatise on the use of rocketry to launch spacecraft. He calculated the speed required for a minimal orbit. In 1928, Herman Potočnik published his book, The Problem of Space Travel — The Rocket Motor. He described the use of orbiting spacecraft for observation of the ground, in a 1945 Wireless World article, the English science fiction writer Arthur C. Clarke described in detail the possible use of communications satellites for mass communications. He suggested that three geostationary satellites would provide coverage over the entire planet, the first artificial satellite was Sputnik 1, launched by the Soviet Union on October 4,1957, and initiating the Soviet Sputnik program, with Sergei Korolev as chief designer. This in turn triggered the Space Race between the Soviet Union and the United States, Sputnik 1 helped to identify the density of high atmospheric layers through measurement of its orbital change and provided data on radio-signal distribution in the ionosphere
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Einstein ring
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This occurs when the source, lens, and observer are all aligned. The first complete Einstein ring, designated B1938+666, was discovered by collaboration between astronomers at the University of Manchester and NASAs Hubble Space Telescope in 1998, gravitational lensing is predicted by Albert Einsteins theory of general relativity. Instead of light from a source traveling in a line, it is bent by the presence of a massive body. An Einstein Ring is a case of gravitational lensing, caused by the exact alignment of the source, lens. This results in a symmetry around the lens, causing a ring-like structure, the size of an Einstein ring is given by the Einstein radius. Note that, over cosmological distances d L S ≠ d S − d L in general, the bending of light by a gravitational body was predicted by Albert Einstein in 1912, a few years before the publication of general relativity in 1916. The ring effect was first mentioned in literature by Orest Chwolson in 1924. Einstein remarked upon this effect in 1936 in a paper prompted by a letter by a Czech engineer, R W Mandl, first, we shall scarcely ever approach closely enough to such a central line. Second, the angle β will defy the power of our instruments. In this statement, β is the Einstein Radius currently denoted by θ E, hundreds of gravitational lenses are currently known. Most rings have been discovered in the radio range, the degree of completeness needed for an image seen through a gravitational lens to qualify as an Einstein ring is yet to be defined. The first Einstein ring was discovered by Hewitt et al. who observed the radio source MG1131+0456 using the Very Large Array. This observation saw a quasar lensed by a galaxy into two separate but very similar images of the same object, the images stretched round the lens into an almost complete ring. These dual images are another possible effect of the source, lens, the first complete Einstein ring to be discovered was B1938+666, which was found by King et al. via optical follow-up with the Hubble Space Telescope of a gravitational lens imaged with MERLIN. As of 2009 this survey has found 85 confirmed gravitational lenses, the Cosmic Horseshoe is a partial Einstein ring which was observed through the gravitational lens of LRG 3-757, a distinctively large Luminous Red Galaxy. It was discovered in 2007 by V. Belokurov et al, another example is the radio/X-Ray Einstein ring around PKS 1830-211, which is unusually strong in radio. It was discovered in X-Ray by Varsha Gupta et al. at the Chandra X-Ray observatory It is also notable for being the first case of a quasar being lensed by an almost face-on spiral galaxy. There is also a ring around galaxy MG1654+1346, the image in the ring is that of a quasar radio lobe
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Tidal range
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The tidal range is the vertical difference between the high tide and the succeeding low tide. Tides are the rise and fall of sea levels caused by the effects of the gravitational forces exerted by the Moon and the Sun. The tidal range is not constant, but changes depending on where the sun and this type of tide is known as a spring tide. During neap tides, when the Moon and Suns gravitational force vectors act in quadrature, neap tides occur during the first and last quarters of the moons phases. The largest annual tidal range can be expected around the time of the equinox, tidal data for coastal areas is published by the national hydrographic service of the country concerned. Tidal data is based on phenomena and is predictable. Storm force winds blowing from a direction for a prolonged time interval combined with low barometric pressure can increase the tidal range. Such weather-related effects on the tide, which can cause ranges in excess of predicted values, the typical tidal range in the open ocean is about 0.6 metres. Closer to the coast, this range is much greater, coastal tidal ranges vary globally and can differ anywhere from near zero to over 11 metres. The exact range depends on the volume of water adjacent to the coast, larger bodies of water have higher ranges, and the geography can act as a funnel amplifying or dispersing the tide. The worlds largest tidal range of 16.3 metres occurs in Bay of Fundy, Canada, the top 50 locations with the largest tidal ranges world-wide are listed by the National Oceanic and Atmospheric Administration of the United States. Some of the smallest tidal ranges occur in the Mediterranean, Baltic, a point within a tidal system where the tidal range is almost zero is called an amphidromic point. The tidal range has been classified as, Micromareal, when the range is lower than 2 metres. Mesomareal, when the range is between 2 metres and 4 metres. Macromareal, when the range is higher than 4 metres
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Tide
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Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon and the Sun and the rotation of the Earth. Some shorelines experience a semi-diurnal tide—two nearly equal high and low tides each day, other locations experience a diurnal tide—only one high and low tide each day. A mixed tide—two uneven tides a day, or one high, Tides vary on timescales ranging from hours to years due to a number of factors. To make accurate records, tide gauges at fixed stations measure water level over time, gauges ignore variations caused by waves with periods shorter than minutes. These data are compared to the level usually called mean sea level. Tidal phenomena are not limited to the oceans, but can occur in other systems whenever a gravitational field varies in time. For example, the part of the Earth is affected by tides. Tide changes proceed via the following stages, Sea level rises over several hours, covering the intertidal zone, the water rises to its highest level, reaching high tide. Sea level falls over several hours, revealing the intertidal zone, the water stops falling, reaching low tide. Oscillating currents produced by tides are known as tidal streams, the moment that the tidal current ceases is called slack water or slack tide. The tide then reverses direction and is said to be turning, slack water usually occurs near high water and low water. But there are locations where the moments of slack tide differ significantly from those of high, Tides are commonly semi-diurnal, or diurnal. The two high waters on a day are typically not the same height, these are the higher high water. Similarly, the two low waters each day are the low water and the lower low water. The daily inequality is not consistent and is small when the Moon is over the equator. From the highest level to the lowest, Highest Astronomical Tide – The highest tide which can be predicted to occur, note that meteorological conditions may add extra height to the HAT. Mean High Water Springs – The average of the two high tides on the days of spring tides, mean High Water Neaps – The average of the two high tides on the days of neap tides. Mean Sea Level – This is the sea level
29.
Tidal force
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The tidal force is a force that is the secondary effect of the force of gravity, it is responsible for the phenomenon of tides. It arises because the force exerted by one body on another is not constant across it. Thus, the force is differential. Consider the gravitational attraction of the Moon on the oceans nearest to the Moon, the solid Earth, there is a mutual attraction between the Moon and the solid Earth, which can be considered to act on its centre of mass. However, the oceans are more strongly attracted and, especially since they are fluid, they approach the Moon slightly. The far oceans are attracted less, viewing the Earth as a whole, we see that all its mass experiences a mutual attraction with that of the Moon, but the near oceans more so than the far oceans, leading to a separation of the two. When a body is acted on by the gravity of another body, Figure 2 shows the differential force of gravity on a spherical body exerted by another body. These so-called tidal forces cause strains on both bodies and may distort them or even, in cases, break one or the other apart. These strains would not occur if the field were uniform, because a uniform field only causes the entire body to accelerate together in the same direction. In the case of a small elastic sphere, the effect of a tidal force is to distort the shape of the body without any change in volume. The sphere becomes an ellipsoid with two bulges, pointing towards and away from the other body, larger objects distort into an ovoid, and are slightly compressed, which is what happens to the Earths oceans under the action of the Moon. The Earth and Moon rotate about their center of mass or barycenter. To an observer on the Earth, very close to this barycenter, all parts of the Earth are subject to the Moons gravitational forces, causing the water in the oceans to redistribute, forming bulges on the sides near the Moon and far from the Moon. When a body rotates while subject to forces, internal friction results in the gradual dissipation of its rotational kinetic energy as heat. In the case for the Earth, and Earths Moon, the loss of kinetic energy results in a gain of about 2 milliseconds per century. If the body is enough to its primary, this can result in a rotation which is tidally locked to the orbital motion. Tidal heating produces dramatic volcanic effects on Jupiters moon Io, stresses caused by tidal forces also cause a regular monthly pattern of moonquakes on Earths Moon. Tidal forces contribute to ocean currents, which moderate global temperatures by transporting heat energy toward the poles and it has been suggested that in addition to other factors, harmonic beat variations in tidal forcing may contribute to climate changes
30.
Earth tide
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Earth tide is the displacement of the solid Earths surface caused by the gravity of the Moon and Sun. Its main component has meter-level amplitude at periods of about 12 hours, the largest body tide constituents are semi-diurnal, but there are also significant diurnal, semi-annual, and fortnightly contributions. Though the gravitational forcing causing earth tides and ocean tides is the same, the larger of the periodic gravitational forces is from the Moon but that of the Sun is also important. The images here show lunar tidal force when the Moon appears directly over 30° N and this pattern remains fixed with the red area directed toward the Moon. Red indicates upward pull, blue downward, if, for example the Moon is directly over 90° W, the red areas are centred on the western northern hemisphere, on upper right. If for example the Moon is directly over 90° W, the centre of the red area is 30° N, 90° W and 30° S, 90° E, at 30° latitude a strong peak occurs once per lunar day, giving a significant diurnal force at that latitude. Along the equator two equally sized peaks are equally sized, giving semi-diurnal force there, the Earth tide encompasses the entire body of the Earth and is unhindered by the thin crust and land masses of the surface, on scales that make the rigidity of rock irrelevant. The oscillation periods of the earth as a whole are not near the astronomical periods, the tide components with a period near twelve hours have a lunar amplitude that are a little more than twice the height of the solar amplitudes, as tabulated below. At new and full moon, the Sun and the Moon are aligned, and the lunar, at first- and third-quarter phases of the moon, lunar and solar tides are perpendicular, and the tidal range is at a minimum. The semi-diurnal tides go through one full cycle about once every 12 hours, the classical theory of Earth tides first became established in 1909, primarily to explain nutations, but are also used in Earth rotation predictions. The semi-diurnal tide is primarily lunar and gives rise to sectorial deformations which rise, sectorial variations of vertical and east-west displacements are maximum at the equator and vanish at the poles. There are two cycles along each latitude, the bulges opposite one another, and the depressions similarly opposed, the diurnal tide is lunisolar, and gives rise to tesseral deformations. The vertical and east-west movement is maximum at 45° latitude and is zero on the equator, finally, fortnightly and semi-annual tides have zonal deformations, as the Moon or Sun gravitation is directed alternately away from the northern and southern hemispheres due to tilt. There is zero vertical displacement at 35°16 latitude, since these displacements affect the vertical direction east-west and north-south variations are often tabulated in milliarcseconds for astronomical use. Displacements caused by ocean tidal loading can exceed the displacements due to the earth body tide, sensitive instruments far inland often have to make similar corrections. Atmospheric loading and storm events may also be measurable, though the masses in movement are less weighty, the amplitudes may vary from those listed within several per cent. Volcanologists use the regular, predictable Earth tide movements to calibrate, the tides may also trigger volcanic events. Seismologists have determined that microseismic events are correlated to tidal variations in Central Asia, the semidiurnal amplitude of terrestrial tides can reach about 55 cm at the equator which is important in GPS, VLBI, and SLR measurements
31.
NASA
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President Dwight D. Eisenhower established NASA in 1958 with a distinctly civilian orientation encouraging peaceful applications in space science. The National Aeronautics and Space Act was passed on July 29,1958, disestablishing NASAs predecessor, the new agency became operational on October 1,1958. Since that time, most US space exploration efforts have led by NASA, including the Apollo Moon landing missions, the Skylab space station. Currently, NASA is supporting the International Space Station and is overseeing the development of the Orion Multi-Purpose Crew Vehicle, the agency is also responsible for the Launch Services Program which provides oversight of launch operations and countdown management for unmanned NASA launches. NASA shares data with various national and international such as from the Greenhouse Gases Observing Satellite. Since 2011, NASA has been criticized for low cost efficiency, from 1946, the National Advisory Committee for Aeronautics had been experimenting with rocket planes such as the supersonic Bell X-1. In the early 1950s, there was challenge to launch a satellite for the International Geophysical Year. An effort for this was the American Project Vanguard, after the Soviet launch of the worlds first artificial satellite on October 4,1957, the attention of the United States turned toward its own fledgling space efforts. This led to an agreement that a new federal agency based on NACA was needed to conduct all non-military activity in space. The Advanced Research Projects Agency was created in February 1958 to develop technology for military application. On July 29,1958, Eisenhower signed the National Aeronautics and Space Act, a NASA seal was approved by President Eisenhower in 1959. Elements of the Army Ballistic Missile Agency and the United States Naval Research Laboratory were incorporated into NASA, earlier research efforts within the US Air Force and many of ARPAs early space programs were also transferred to NASA. In December 1958, NASA gained control of the Jet Propulsion Laboratory, NASA has conducted many manned and unmanned spaceflight programs throughout its history. Some missions include both manned and unmanned aspects, such as the Galileo probe, which was deployed by astronauts in Earth orbit before being sent unmanned to Jupiter, the experimental rocket-powered aircraft programs started by NACA were extended by NASA as support for manned spaceflight. This was followed by a space capsule program, and in turn by a two-man capsule program. This goal was met in 1969 by the Apollo program, however, reduction of the perceived threat and changing political priorities almost immediately caused the termination of most of these plans. NASA turned its attention to an Apollo-derived temporary space laboratory, to date, NASA has launched a total of 166 manned space missions on rockets, and thirteen X-15 rocket flights above the USAF definition of spaceflight altitude,260,000 feet. The X-15 was an NACA experimental rocket-powered hypersonic research aircraft, developed in conjunction with the US Air Force, the design featured a slender fuselage with fairings along the side containing fuel and early computerized control systems
