1.
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
2.
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
3.
Hilda group
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The Hilda or Hildian asteroids are a dynamical group of asteroids in a 3,2 orbital resonance with Jupiter. The namesake is the asteroid 153 Hilda, Hildas move in their elliptical orbits so that their aphelia put them opposite Jupiter, or 60 degrees ahead of or behind Jupiter at the L4 and L5 Lagrangian points. Over three successive orbits each Hilda asteroid approaches all of three points in sequence. A Hildas orbit has an axis between 3.7 AU and 4.2 AU, an eccentricity less than 0.3. Two collisional families exist within the Hilda group, the Hilda family, the namesake for the latter family is 1911 Schubart. There are more than 1,100 known Hilda asteroids including unnumbered objects, Hildas surface colors often correspond to the low-albedo D-type and P-type, however, a small portion are C-type. D-type and P-type asteroids have surface colors, and thus also surface mineralogies and this implies that they share a common origin. The asteroids of the Hilda group are in 3,2 mean-motion resonance with Jupiter and that is, their orbital periods are 2/3 that of Jupiter. They move along the orbits with a semimajor axis near 4.0 AU and moderate values of eccentricity, unlike the Jupiter trojans they may have any difference in longitude with Jupiter, nevertheless avoiding dangerous approaches to the planet. The Hildas taken together constitute a dynamic triangular figure with slightly convex sides, the asteroidal stream within the sides of the triangle is about 1 AU wide, and in the apexes this value is 20-40% greater. Figure 1 shows the positions of the Hildas against a background of all known asteroids up to Jupiters orbit at January 1,2005, each of the Hilda objects moves along its own elliptic orbit. However, at any moment the Hildas together constitute this triangular configuration, figure 2 illustrates this with the Hildas positions against a background of their orbits. For the majority of asteroids, their position in orbit may be arbitrary, except for the external parts of the apexes. The Hildas Triangle has proven to be stable over a long time span. The typical Hilda object has a retrograde perihelion motion, on average, the velocity of perihelion motion is greater when the orbital eccentricity is lesser, while the nodes move more slowly. Moreover, the apsidal line oscillates near the line of conjunction with different amplitude, in addition to the fact that the Hildas triangle revolves in sync with Jupiter, the density of asteroids in the stream exhibits quasi-periodical waves. At any time, the density of objects in the triangles apexes is more than twice the density within the sides. The Hildas rest at their aphelia in the apexes for an average of 5. 0-5.5 years, whereas they move along the more quickly
4.
Asteroid belt
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The asteroid belt is the circumstellar disc in the Solar System located roughly between the orbits of the planets Mars and Jupiter. It is occupied by numerous irregularly shaped bodies called asteroids or minor planets, the asteroid belt is also termed the main asteroid belt or main belt to distinguish it from other asteroid populations in the Solar System such as near-Earth asteroids and trojan asteroids. About half the mass of the belt is contained in the four largest asteroids, Ceres, Vesta, Pallas, the total mass of the asteroid belt is approximately 4% that of the Moon, or 22% that of Pluto, and roughly twice that of Plutos moon Charon. Ceres, the belts only dwarf planet, is about 950 km in diameter, whereas Vesta, Pallas. The remaining bodies range down to the size of a dust particle, the asteroid material is so thinly distributed that numerous unmanned spacecraft have traversed it without incident. Nonetheless, collisions between large asteroids do occur, and these can form a family whose members have similar orbital characteristics. Individual asteroids within the belt are categorized by their spectra. The asteroid belt formed from the solar nebula as a group of planetesimals. Planetesimals are the precursors of the protoplanets. Between Mars and Jupiter, however, gravitational perturbations from Jupiter imbued the protoplanets with too much energy for them to accrete into a planet. Collisions became too violent, and instead of fusing together, the planetesimals, as a result,99. 9% of the asteroid belts original mass was lost in the first 100 million years of the Solar Systems history. Some fragments eventually found their way into the inner Solar System, Asteroid orbits continue to be appreciably perturbed whenever their period of revolution about the Sun forms an orbital resonance with Jupiter. At these orbital distances, a Kirkwood gap occurs as they are swept into other orbits. Classes of small Solar System bodies in other regions are the objects, the centaurs, the Kuiper belt objects, the scattered disc objects, the sednoids. On 22 January 2014, ESA scientists reported the detection, for the first definitive time, of water vapor on Ceres, the detection was made by using the far-infrared abilities of the Herschel Space Observatory. The finding was unexpected because comets, not asteroids, are considered to sprout jets. According to one of the scientists, The lines are becoming more and more blurred between comets and asteroids. This pattern, now known as the Titius–Bode law, predicted the semi-major axes of the six planets of the provided one allowed for a gap between the orbits of Mars and Jupiter
5.
Orbit
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In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet about a star or a natural satellite around a planet. Normally, orbit refers to a regularly repeating path around a body, to a close approximation, planets and satellites follow elliptical orbits, with the central mass being orbited at a focal point of the ellipse, as described by Keplers laws of planetary motion. For ease of calculation, in most situations orbital motion is adequately approximated by Newtonian Mechanics, historically, the apparent motions of the planets were described by European and Arabic philosophers using the idea of celestial spheres. This model posited the existence of perfect moving spheres or rings to which the stars and it assumed the heavens were fixed apart from the motion of the spheres, and was developed without any understanding of gravity. After the planets motions were accurately measured, theoretical mechanisms such as deferent. Originally geocentric it was modified by Copernicus to place the sun at the centre to help simplify the model, the model was further challenged during the 16th century, as comets were observed traversing the spheres. The basis for the understanding of orbits was first formulated by Johannes Kepler whose results are summarised in his three laws of planetary motion. Second, he found that the speed of each planet is not constant, as had previously been thought. Third, Kepler found a relationship between the orbital properties of all the planets orbiting the Sun. For the planets, the cubes of their distances from the Sun are proportional to the squares of their orbital periods. Jupiter and Venus, for example, are respectively about 5.2 and 0.723 AU distant from the Sun, their orbital periods respectively about 11.86 and 0.615 years. The proportionality is seen by the fact that the ratio for Jupiter,5. 23/11.862, is equal to that for Venus,0. 7233/0.6152. Idealised orbits meeting these rules are known as Kepler orbits, isaac Newton demonstrated that Keplers laws were derivable from his theory of gravitation and that, in general, the orbits of bodies subject to gravity were conic sections. Newton showed that, for a pair of bodies, the sizes are in inverse proportion to their masses. Where one body is more massive than the other, it is a convenient approximation to take the center of mass as coinciding with the center of the more massive body. Lagrange developed a new approach to Newtonian mechanics emphasizing energy more than force, in a dramatic vindication of classical mechanics, in 1846 le Verrier was able to predict the position of Neptune based on unexplained perturbations in the orbit of Uranus. This led astronomers to recognize that Newtonian mechanics did not provide the highest accuracy in understanding orbits, in relativity theory, orbits follow geodesic trajectories which are usually approximated very well by the Newtonian predictions but the differences are measurable. Essentially all the evidence that can distinguish between the theories agrees with relativity theory to within experimental measurement accuracy
6.
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
7.
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
8.
Trojan War
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In Greek mythology, the Trojan War was waged against the city of Troy by the Achaeans after Paris of Troy took Helen from her husband Menelaus, king of Sparta. The war is one of the most important events in Greek mythology and has been narrated through many works of Greek literature, most notably through Homers Iliad. The Iliad relates four days in the year of the decade-long siege of Troy. Other parts of the war are described in a cycle of epic poems, episodes from the war provided material for Greek tragedy and other works of Greek literature, and for Roman poets including Virgil and Ovid. Zeus sent the goddesses to Paris, who judged that Aphrodite, as the fairest, in exchange, Aphrodite made Helen, the most beautiful of all women and wife of Menelaus, fall in love with Paris, who took her to Troy. Agamemnon, king of Mycenae and the brother of Helens husband Menelaus, led an expedition of Achaean troops to Troy and besieged the city for ten years because of Paris insult. After the deaths of heroes, including the Achaeans Achilles and Ajax, and the Trojans Hector and Paris. The Achaeans slaughtered the Trojans and desecrated the temples, thus earning the gods wrath, few of the Achaeans returned safely to their homes and many founded colonies in distant shores. The Romans later traced their origin to Aeneas, one of the Trojans, in 1868, however, the German archaeologist Heinrich Schliemann met Frank Calvert, who convinced Schliemann that Troy was a real city at what is now Hissarlik in Turkey. On the basis of excavations conducted by Schliemann and others, this claim is now accepted by most scholars, whether there is any historical reality behind the Trojan War remains an open question. The events of the Trojan War are found in works of Greek literature. There is no single, authoritative text which tells the events of the war. Instead, the story is assembled from a variety of sources, the most important literary sources are the two epic poems traditionally credited to Homer, the Iliad and the Odyssey, composed sometime between the 9th and 6th centuries BC. Each poem narrates only a part of the war, the Iliad covers a short period in the last year of the siege of Troy, while the Odyssey concerns Odysseuss return to his home island of Ithaca, following the sack of Troy. Other parts of the Trojan War were told in the poems of the Epic Cycle, also known as the Cyclic Epics, the Cypria, Aethiopis, Little Iliad, Iliou Persis, Nostoi, and Telegony. Though these poems survive only in fragments, their content is known from an included in Proclus Chrestomathy. The authorship of the Cyclic Epics is uncertain, both the Homeric epics and the Epic Cycle take origin from oral tradition. Even after the composition of the Iliad, Odyssey, and the Cyclic Epics, events and details of the story that are only found in later authors may have been passed on through oral tradition and could be as old as the Homeric poems
9.
Libration
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Lunar libration is distinct from the slight changes in the Moons visual size as seen from Earth. Although this appearance can also be described as a motion, libration is caused by actual changes in the physical distance of the Moon. The Moon generally has one hemisphere facing the Earth, due to tidal locking, therefore, humans first view of the far side of the Moon resulted from lunar exploration on October 7,1959. However, this picture is only approximately true, over time. Libration is manifested as a slow rocking back and forth of the Moon as viewed from Earth, libration in latitude results from a slight inclination between the Moons axis of rotation and the normal to the plane of its orbit around Earth. Its origin is analogous to how the seasons arise from Earths revolution about the Sun. In 1772 Lagranges analyses determined that small bodies can stably share the orbit as a planet if they remain near Lagrange points. Such ‘trojan asteroids’ have been found co-orbiting with Earth, Jupiter, Mars, trojan asteroids associated with Earth are difficult to observe in the visible spectrum, as their libration paths are such that they would be visible primarily in the daylight sky. In 2010, however, using infrared observation techniques, the asteroid 2010 TK7 was found to be a companion of the Earth, it librates around the leading Lagrange point, L4
10.
Lagrangian point
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The Lagrange points mark positions where the combined gravitational pull of the two large masses provides precisely the centrifugal force required to orbit with them. There are five points, labeled L1 to L5, all in the orbital plane of the two large bodies. The first three are on the line connecting the two bodies, the last two, L4 and L5, each form an equilateral triangle with the two large bodies. The two latter points are stable, which implies that objects can orbit around them in a coordinate system tied to the two large bodies. Several planets have satellites near their L4 and L5 points with respect to the Sun, the three collinear Lagrange points were discovered by Leonhard Euler a few years before Lagrange discovered the remaining two. In 1772, Joseph-Louis Lagrange published an Essay on the three-body problem, in the first chapter he considered the general three-body problem. From that, in the chapter, he demonstrated two special constant-pattern solutions, the collinear and the equilateral, for any three masses, with circular orbits. The five Lagrangian points are labeled and defined as follows, The L1 point lies on the line defined by the two large masses M1 and M2, and between them. It is the most intuitively understood of the Lagrangian points, the one where the attraction of M2 partially cancels M1s gravitational attraction. Explanation An object that orbits the Sun more closely than Earth would normally have an orbital period than Earth. If the object is directly between Earth and the Sun, then Earths gravity counteracts some of the Suns pull on the object, the closer to Earth the object is, the greater this effect is. At the L1 point, the period of the object becomes exactly equal to Earths orbital period. L1 is about 1.5 million kilometers from Earth, the L2 point lies on the line through the two large masses, beyond the smaller of the two. Here, the forces of the two large masses balance the centrifugal effect on a body at L2. Explanation On the opposite side of Earth from the Sun, the period of an object would normally be greater than that of Earth. The extra pull of Earths gravity decreases the orbital period of the object, like L1, L2 is about 1.5 million kilometers from Earth. The L3 point lies on the line defined by the two masses, beyond the larger of the two. Explanation L3 in the Sun–Earth system exists on the side of the Sun
11.
Semi-major and semi-minor axes
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In geometry, the major axis of an ellipse is its longest diameter, a line segment that runs through the center and both foci, with ends at the widest points of the perimeter. The semi-major axis is one half of the axis, and thus runs from the centre, through a focus. Essentially, it is the radius of an orbit at the two most distant points. For the special case of a circle, the axis is the radius. One can think of the axis as an ellipses long radius. The semi-major axis of a hyperbola is, depending on the convention, thus it is the distance from the center to either vertex of the hyperbola. A parabola can be obtained as the limit of a sequence of ellipses where one focus is fixed as the other is allowed to move arbitrarily far away in one direction. Thus a and b tend to infinity, a faster than b, the semi-minor axis is a line segment associated with most conic sections that is at right angles with the semi-major axis and has one end at the center of the conic section. It is one of the axes of symmetry for the curve, in an ellipse, the one, in a hyperbola. The semi-major axis is the value of the maximum and minimum distances r max and r min of the ellipse from a focus — that is. In astronomy these extreme points are called apsis, the semi-minor axis of an ellipse is the geometric mean of these distances, b = r max r min. The eccentricity of an ellipse is defined as e =1 − b 2 a 2 so r min = a, r max = a. Now consider the equation in polar coordinates, with one focus at the origin, the mean value of r = ℓ / and r = ℓ /, for θ = π and θ =0 is a = ℓ1 − e 2. In an ellipse, the axis is the geometric mean of the distance from the center to either focus. The semi-minor axis of an ellipse runs from the center of the ellipse to the edge of the ellipse, the semi-minor axis is half of the minor axis. The minor axis is the longest line segment perpendicular to the axis that connects two points on the ellipses edge. The semi-minor axis b is related to the axis a through the eccentricity e. A parabola can be obtained as the limit of a sequence of ellipses where one focus is fixed as the other is allowed to move arbitrarily far away in one direction
12.
Max Wolf
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Maximilian Franz Joseph Cornelius Max Wolf was a German astronomer and a pioneer in the field of astrophotography. He was Chairman of Astronomy at the University of Heidelberg and director of the Heidelberg-Königstuhl State Observatory from 1902 until his death and he was born in Heidelberg, Germany on June 21,1863, the son of a popular medical doctor, Dr. Franz Wolf. His father encouraged an interest in science and built an observatory for his son in the garden of the family home and it is from here that Wolf is credited with his first astronomical discovery, comet 14P/Wolf, in 1884. He attended the world famous university and, in 1888, at the age of 25. He spent one year of study in Stockholm, the only significant time he would spend outside of Heidelberg in his life. He returned to the University of Heidelberg and accepted the position of privat-docent in 1890, a popular lecturer in astronomy, he declined offers of positions from other institutions. In 1902 he was appointed Chair of Astronomy and Director of the new Landessternwarte Heidelberg-Königstuhl observatory, positions he would hold until his death in 1932. While the new observatory was being built Wolf was appointed to supervise the construction and he proved to be a not only a capable supervisor but also a successful money raiser. Wolf immediately designed and ordered a double refractor telescope from American astronomer and instrument builder and this instrument, known as the Bruce double-astrograph, with parallel 16 in lenses and a fast f/5 focal ratio, became the observatorys primary research telescope. He also raised money for a 28 in reflector telescope, the first for the observatory, in 1910 Wolf proposed to the Carl Zeiss optics firm the creation of a new instrument, now known as the planetarium. World War I intervened before this could be developed, but the Carl Zeiss company resumed this project after peace was restored, the first official public showing was at the Deutsches Museum in Munich, Germany on October 21,1923. During his trip to America he was interested in learning more about the new field of astrophotography. The two would become lifelong correspondents, competitors, collaborators and friends, Wolf was clearly moved by the death of his friend in 1923 and wrote a long obituary. The University, already world-renowned in many fields, became well known for astronomy. Wolf himself was a researcher, contributing numerous papers in many areas of astronomy up to the end of his life. Like his friend, E. E. Barnard, he died rather young for an astronomer and he died in Heidelberg on October 3,1932, at the age of 69. He was survived by his widow and three sons, Wolf started his career as a comet hunter and continued to discover them throughout his life. He discovered or co-discovered several comets, including 14P/Wolf and 43P/Wolf-Harrington and he won a competition with E. E. Barnard on who would be the first to observe the return of Halleys Comet in April,1910
13.
Asteroid family
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An asteroid family is a population of asteroids that share similar proper orbital elements, such as semimajor axis, eccentricity, and orbital inclination. The members of the families are thought to be fragments of past asteroid collisions, an asteroid family is a more specific term than asteroid group whose members, while sharing some broad orbital characteristics, may be otherwise unrelated to each other. Large prominent families contain several hundred recognized asteroids, small, compact families can have only about ten identified members. About 33% to 35% of asteroids in the belt are family members. There are about 20 to 30 reliably recognized families, with tens of less certain groupings. One family has been identified associated with the dwarf planet Haumea, some studies have tried to find evidence of collisional families among the trojan asteroids, but at present the evidence is inconclusive. The families are thought to form as a result of collisions between asteroids, in many or most cases the parent body was shattered, but there are also several families which resulted from a large cratering event which did not disrupt the parent body. Such cratering families typically consist of a large body and a swarm of asteroids that are much smaller. Some families have complex structures which are not satisfactorily explained at the moment. Due to the method of origin, all the members have closely matching compositions for most families, notable exceptions are those families which formed from a large differentiated parent body. Asteroid families are thought to have lifetimes of the order of a billion years and this is significantly shorter than the Solar Systems age, so few if any are relics of the early Solar System. Such small asteroids then become subject to such as the Yarkovsky effect that can push them towards orbital resonances with Jupiter over time. Once there, they are relatively rapidly ejected from the asteroid belt, tentative age estimates have been obtained for some families, ranging from hundreds of millions of years to less than several million years as for the compact Karin family. Old families are thought to contain few small members, and this is the basis of the age determinations and it is supposed that many very old families have lost all the smaller and medium-sized members, leaving only a few of the largest intact. A suggested example of old family remains are the 9 Metis and 113 Amalthea pair. Further evidence for a number of past families comes from analysis of chemical ratios in iron meteorites. These show that there must have once been at least 50 to 100 parent bodies large enough to be differentiated, when the orbital elements of main belt asteroids are plotted, a number of distinct concentrations are seen against the rather uniform background distribution of generic asteroids. These concentrations are the asteroid families, the proper elements are related constants of motion that remain almost constant for times of at least tens of millions of years, and perhaps longer
14.
Spectrum
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A spectrum is a condition that is not limited to a specific set of values but can vary, without steps, across a continuum. The word was first used scientifically in optics to describe the rainbow of colors in visible light after passing through a prism, as scientific understanding of light advanced, it came to apply to the entire electromagnetic spectrum. Spectrum has since been applied by analogy to topics outside of optics, thus, one might talk about the spectrum of political opinion, or the spectrum of activity of a drug, or the autism spectrum. In these uses, values within a spectrum may not be associated with precisely quantifiable numbers or definitions, such uses imply a broad range of conditions or behaviors grouped together and studied under a single title for ease of discussion. Nonscientific uses of the spectrum are sometimes misleading. For instance, a single left–right spectrum of opinion does not capture the full range of peoples political beliefs. Political scientists use a variety of biaxial and multiaxial systems to accurately characterize political opinion. In most modern usages of spectrum there is a theme between the extremes at either end. This was not always true in older usage, in Latin spectrum means image or apparition, including the meaning spectre. Spectral evidence is testimony about what was done by spectres of persons not present physically and it was used to convict a number of persons of witchcraft at Salem, Massachusetts in the late 17th century. The word spectrum was used to designate a ghostly optical afterimage by Goethe in his Theory of Colors and Schopenhauer in On Vision. The prefix spectro- is used to form words relating to spectra, for example, a spectrometer is a device used to record spectra and spectroscopy is the use of a spectrometer for chemical analysis. In the 17th century the word spectrum was introduced into optics by Isaac Newton, soon the term referred to a plot of light intensity or power as a function of frequency or wavelength, also known as a spectral density plot. The term spectrum was expanded to apply to other waves, such as sound waves that could also be measured as a function of frequency, frequency spectrum and power spectrum of a signal. The term now applies to any signal that can be measured or decomposed along a variable such as energy in electron spectroscopy or mass to charge ratio in mass spectrometry. Spectrum is also used to refer to a representation of the signal as a function of the dependent variable. Devices used to measure an electromagnetic spectrum are called spectrograph or spectrometer, the visible spectrum is the part of the electromagnetic spectrum that can be seen by the human eye. The wavelength of light ranges from 390 to 700 nm
15.
Tholin
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Tholins are a class of heteropolymer molecules formed by solar ultraviolet irradiation of simple organic compounds such as methane or ethane, often in combination with nitrogen. They usually have a reddish-brown appearance, however, we have recently succeeded, through sequential and non-sequential pyrolysis followed by gas chromatography-mass spectrometry in determining something of the composition of this material. It is clearly not a repetition of the same monomeric unit—and some other term is needed. We propose, as a descriptive term, ‘tholins’, although we were tempted by the phrase ‘star-tar’. Tholins are not one specific compound but rather are descriptive of a spectrum of molecules that give a reddish, as illustrated to the right, tholins are believed to form through a chain of chemical reactions. This begins with the dissociation and ionization of molecular nitrogen and methane by energetic particles and this is followed by the formation of ethylene, ethane, acetylene, hydrogen cyanide, and other small simple molecules and small positive ions. These atmospherically-derived substances are distinct from ice tholin, which is formed instead by irradiation of clathrates of water, the surfaces of comets, centaurs, and many icy moons and Kuiper-belt objects in the outer solar system are rich in deposits of tholins. Titan tholins are nitrogen-rich organic substances produced by the irradiation of the mixtures of nitrogen and methane found in the atmosphere. Titans atmosphere is 98. 4% nitrogen and the remaining 1. 6% composed of methane, in the case of Titan, the haze and orange-red color of its atmosphere is thought to be caused by the presence of tholins. Neptunes moon Triton is observed to have the color characteristic of tholins. Tritons atmosphere is 99. 9% nitrogen and 0. 1% methane, tholins also occur on the dwarf planet Pluto and its moon Charon and are responsible for their red colors as well as the blue tint of Plutos atmosphere. Tholins have also detected on the plutino Ixion. The HR4796 system is approximately 220 light years from Earth, some researchers have speculated that Earth may have been seeded by organic compounds early in its development by tholin-rich comets, providing the raw material necessary for life to develop. Tholins do not exist naturally on present-day Earth due to the character of the free oxygen component of its atmosphere ever since the Great Oxygenation Event around 2.4 billion years ago. Tholins can act as a screen for protecting planetary surfaces from ultraviolet radiation. A wide variety of bacteria are able to use tholins as their sole source of carbon. Tholins could have been the first microbial food for heterotrophic microorganisms before autotrophy evolved
16.
Binary star
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A binary star is a star system consisting of two stars orbiting around their common barycenter. Systems of two or more stars are called multiple star systems and these systems, especially when more distant, often appear to the unaided eye as a single point of light, and are then revealed as multiple by other means. Research over the last two centuries suggests that half or more of visible stars are part of star systems. The term double star is used synonymously with binary star, however. Optical doubles are so called because the two stars close together in the sky as seen from the Earth, they are almost on the same line of sight. Nevertheless, their doubleness depends only on this effect, the stars themselves are distant from one another. A double star can be revealed as optical by means of differences in their measurements, proper motions. Most known double stars have not been studied closely to determine whether they are optical doubles or they are doubles physically bound through gravitation into a multiple star system. This also determines an empirical mass-luminosity relationship from which the masses of stars can be estimated. Binary stars are often detected optically, in case they are called visual binaries. Many visual binaries have long periods of several centuries or millennia. They may also be detected by indirect techniques, such as spectroscopy or astrometry, if components in binary star systems are close enough they can gravitationally distort their mutual outer stellar atmospheres. In some cases, these close binary systems can exchange mass, examples of binaries are Sirius, and Cygnus X-1. Binary stars are common as the nuclei of many planetary nebulae. This should be called a double star, and any two stars that are thus mutually connected, form the binary sidereal system which we are now to consider. By the modern definition, the binary star is generally restricted to pairs of stars which revolve around a common center of mass. Binary stars which can be resolved with a telescope or interferometric methods are known as visual binaries, for most of the known visual binary stars one whole revolution has not been observed yet, they are observed to have travelled along a curved path or a partial arc. The more general term double star is used for pairs of stars which are seen to be together in the sky
17.
Formation and evolution of the Solar System
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The formation of the Solar System began 4.6 billion years ago with the gravitational collapse of a small part of a giant molecular cloud. This model, known as the hypothesis, was first developed in the 18th century by Emanuel Swedenborg, Immanuel Kant. Its subsequent development has interwoven a variety of disciplines including astronomy, physics, geology. Since the dawn of the age in the 1950s and the discovery of extrasolar planets in the 1990s. The Solar System has evolved considerably since its initial formation, many moons have formed from circling discs of gas and dust around their parent planets, while other moons are thought to have formed independently and later been captured by their planets. Still others, such as Earths Moon, may be the result of giant collisions, collisions between bodies have occurred continually up to the present day and have been central to the evolution of the Solar System. The positions of the planets often shifted due to gravitational interactions and this planetary migration is now thought to have been responsible for much of the Solar Systems early evolution. In the far distant future, the gravity of passing stars will gradually reduce the Suns retinue of planets, some planets will be destroyed, others ejected into interstellar space. Ultimately, over the course of tens of billions of years, the first step toward a theory of Solar System formation and evolution was the general acceptance of heliocentrism, which placed the Sun at the centre of the system and the Earth in orbit around it. This concept had developed for millennia, but was not widely accepted until the end of the 17th century, the first recorded use of the term Solar System dates from 1704. The most significant criticism of the hypothesis was its apparent inability to explain the Suns relative lack of momentum when compared to the planets. However, since the early 1980s studies of stars have shown them to be surrounded by cool discs of dust and gas, exactly as the nebular hypothesis predicts. Understanding of how the Sun is expected to continue to evolve required an understanding of the source of its power, in 1935, Eddington went further and suggested that other elements also might form within stars. Fred Hoyle elaborated on this premise by arguing that evolved stars called red giants created many elements heavier than hydrogen, when a red giant finally casts off its outer layers, these elements would then be recycled to form other star systems. The nebular hypothesis says that the Solar System formed from the collapse of a fragment of a giant molecular cloud. The cloud was about 20 parsec across, while the fragments were roughly 1 parsec across, the further collapse of the fragments led to the formation of dense cores 0. 01–0.1 pc in size. One of these fragments formed what became the Solar System. The remaining 2% of the mass consisted of elements that were created by nucleosynthesis in earlier generations of stars
18.
Planetary migration
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Planetary migration is the most likely explanation for hot Jupiters, extrasolar planets with jovian masses, but orbits of only a few days. It has also become clear that terrestrial-mass planets may be subject to rapid inward migration if they form while the gas disk is still present and this may affect the formation of the cores of the giant planets, if those planets form via the core accretion mechanism. Protoplanetary gas disks around stars are observed to have lifetimes of a few million years. Although the sense of migration is typically inwards in locally isothermal disks, during the late phase of planetary system formation, massive protoplanets and planetesimals gravitationally interact in a chaotic manner causing many planetesimals to be thrown into new orbits. This results in angular-momentum exchange between the planets and the planetesimals, and leads to migration, outward migration of Neptune is believed to be responsible for the resonant capture of Pluto and other Plutinos into the 3,2 resonance with Neptune. This type of orbital migration arises from the force exerted by a sufficiently massive body embedded in a disk on the surrounding disks gas. By the reaction principle of mechanics, the gas exerts an equal and opposite gravitational force on the body. This torque alters the angular momentum of the orbit, resulting in a variation of the orbital elements. An increase over time of the axis leads to outward migration, i. e. away from the star. Small planets undergo Type I migration driven by torques arising from waves launched at the locations of the Lindblad, Lindblad resonances excite spiral density waves in the surrounding gas, interior and exterior of the planets orbit. In most cases, the spiral wave exerts a greater torque than does the inner wave, causing the planet to lose angular momentum. Additional co-rotation torques are also exerted by gas orbiting with a similar to that of the planet. In a reference frame attached to the planet, this gas follows horseshoe orbits, the gas reversing course ahead of the planet originates from a larger semi-major axis and may be cooler and denser than the gas reversing course behind the planet. This may result in a region of excess density ahead of the planet and of lesser density behind the planet, causing the planet to gain angular momentum. The planet mass for which migration can be approximated to Type I depends on the gas pressure and, to a lesser extent. In locally isothermal disks and far from steep density and temperature gradients, regions of outward migration may exist for some planetary mass ranges and disk conditions in both local isothermal and non-isothermal disks. Type I migration in a local isothermal disk was shown to be compatible with the formation and evolution of some of the observed Kepler planets, the accretion of material by the planet also produces a heating torque that causes the planet to gain angular momentum. A planet massive enough to open a gap in a gaseous disk undergoes a regime of migration referred to as Type II, the gravity of a massive planet exerts a torque on the nearby gas, repelling it from its orbit
19.
Trojan (astronomy)
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In astronomy, a trojan is a minor planet or moon that shares the orbit of a planet or larger moon, wherein the trojan remains in the same, stable position relative to the larger object. In particular, a trojan remains near one of the two points of stability – designated L4 and L5 – which lie approximately 60° ahead of and behind the larger body. Trojan points make up two of five types of Lagrangian points, and a trojan is a type of Lagrangian object and they are one type of co-orbital object. In this arrangement, the star and the smaller planet orbit about their common barycenter. A much smaller mass located at one of the Lagrangian points is subject to a gravitational force that acts through this barycenter. Hence the object can orbit around the barycenter with the orbital period as the planet. The Jupiter trojans account for most known trojans in the Solar System and they are divided into the Greek camp in front of and the Trojan camp trailing behind Jupiter in their orbit. Numerical calculations of the dynamics involved indicate that Saturn and Uranus probably do not have any primordial trojans. The discovery of the first Earth trojan,2010 TK7, was announced by NASA in 2011, unlike trojan minor planets that share the orbit with a planet, a trojan moon is a moon orbiting near the trojan point of another, larger moon. All known trojan moons are part of the Saturn system, telesto and Calypso are trojans of Tethys, and Helene and Polydeuces of Dione. There is also a concept of a trojan planet, a planet that orbits at the trojan point of another, larger planet. Such a pair of co-orbital exoplanets was already thought to exist in another star system, in 1772, the Italian–French mathematician and astronomer Joseph-Louis Lagrange obtained two constant-pattern solutions of the general three-body problem. In the restricted problem, with one mass negligible, the five possible positions of that mass are now termed Lagrangian points. The term trojan originally referred to the asteroids that orbit close to the Lagrangian points of Jupiter. These have long been named after characters from the Trojan War of Greek mythology, there are two exceptions, which were named before the convention was put in place, the Greek-themed 617 Patroclus and the Trojan-themed 624 Hektor, which were assigned to the wrong sides. Astronomers estimate that the Jupiter trojans are about as numerous as the asteroids of the asteroid belt, later on, objects were found orbiting near the Lagrangian points of Neptune, Mars, Earth, Uranus, and Venus. Minor planets at the Lagrangian points of other than Jupiter may be called Lagrangian minor planets. 17 Neptune trojans are known, but large Neptune trojans are expected to outnumber the large Jupiter trojans by an order of magnitude,2010 TK7 was confirmed to be the first known Earth trojan in 2011
20.
Small Solar System body
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A Small Solar System Body is an object in the Solar System that is neither a planet, nor a dwarf planet, nor a natural satellite. The term was first defined in 2006 by the International Astronomical Union, all other objects, except satellites, orbiting the Sun shall be referred to collectively as Small Solar System Bodies. These currently include most of the Solar System asteroids, most Trans-Neptunian Objects, comets and this encompasses all comets and all minor planets other than those that are dwarf planets. Except for the largest, which are in equilibrium, natural satellites differ from small Solar System bodies not in size. The orbits of satellites are not centered on the Sun, but around other Solar System objects such as planets, dwarf planets. Some of the larger small Solar System bodies may be reclassified in future as dwarf planets, the orbits of the vast majority of small Solar System bodies are located in two distinct areas, namely the asteroid belt and the Kuiper belt. These two belts possess some internal structure related to perturbations by the planets, and have fairly loosely defined boundaries. Other areas of the Solar System also encompass small bodies in smaller concentrations and these include the near-Earth asteroids, centaurs, comets, and scattered disc objects
21.
Mars trojan
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The Mars trojans are a group of objects that share the orbit of the planet Mars around the Sun. They can be found around the two Lagrangian points 60° ahead of and behind Mars, the origin of the Mars trojans is not well understood. One theory suggests that they were captured in its Lagrangian points as the Solar System was forming, however, spectral studies of the Mars trojans indicate this may not be the case. One explanation for this involves asteroids wandering into the Mars Lagrangian points later in the Solar Systems formation and this is also questionable considering the very low mass of Mars
22.
Neptune trojan
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Neptune trojans are bodies in orbit around the Sun that orbit near one of the stable Lagrangian points of Neptune. They therefore have approximately the same period as Neptune and follow roughly the same orbital path. Seventeen Neptune trojans are known, of which thirteen orbit near the Sun–Neptune L4 Lagrangian point 60° ahead of Neptune. The Neptune trojans are termed trojans by analogy with the Jupiter trojans and it is suspected that large Neptune trojans could outnumber Jupiter trojans by an order of magnitude. In 2010, the discovery of the first known L5 Neptune trojan,2008 LC18, was announced, Neptunes trailing L5 region is currently very difficult to observe because it is along the line-of-sight to the center of the Milky Way, an area of the sky crowded with stars. However, New Horizons may not have had sufficient downlink bandwidth, in 2001, the first Neptune trojan was discovered,2001 QR322, near Neptunes L4 region, and with it the fifth known populated stable reservoir of small bodies in the Solar System. In 2005, the discovery of the high-inclination trojan 2005 TN53 has indicated that the Neptune trojans populate thick clouds, on August 12,2010, the first L5 trojan,2008 LC18, was announced. It was discovered by a survey that scanned regions where the light from the stars near the Galactic Center is obscured by dust clouds. This suggests that large L5 trojans are as common as large L4 trojans, to within uncertainty and it would have been possible for the New Horizons spacecraft to investigate L5 Neptune trojans discovered by 2014, when it passed through this region of space en route to Pluto. Some of the patches where the light from the Galactic Center is obscured by dust clouds are along New Horizonss flight path, allowing detection of objects that the spacecraft could image. 2011 HM102, the highest-inclination Neptune trojan known, was just bright enough for New Horizons to observe it in end-2013 at a distance of 1.2 AU. However, New Horizons may not have had sufficient downlink bandwidth, the orbits of Neptune trojans are highly stable, Neptune may have retained up to 50% of the original post-migration trojan population over the age of the Solar System. Neptunes L5 can host stable trojans equally well as its L4, Neptune trojans can librate up to 30° from their associated Lagrangian points with a 10, 000-year period. Neptune trojans that escape enter orbits similar to centaurs, although Neptune cannot currently capture stable trojans, roughly 2. 8% of the centaurs within 34 AU are predicted to be Neptune co-orbitals. Of these, 54% would be in horseshoe orbits, 10% would be quasi-satellites, the unexpected high-inclination trojans are the key to understanding the origin and evolution of the population as a whole. The existence of high-inclination Neptune trojans points to a capture during planetary migration instead of in situ or collisional formation. The estimated equal number of large L5 and L4 trojans indicates that there was no gas drag during capture, the capture of Neptune trojans during a migration of the planets occurs via process similar to the chaotic capture of Jupiter trojans in the Nice model. When Uranus and Neptune are near but not in a mean-motion resonance the period at which the locations where Uranus passes Neptune circulate can resonate with the periods of Neptune trojans
23.
Co-orbital configuration
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There are several classes of co-orbital objects, depending on their point of libration. The most common and best-known class is the trojan, which librates around one of the two stable Lagrangian points, L4 and L5, 60° ahead of and behind the larger body respectively. Another class is the orbit, in which objects librate around 180° from the larger body. Objects librating around 0° are called quasi-satellites, an exchange orbit occurs when two co-orbital objects are of similar masses and thus exert a non-negligible influence on each other. The objects can exchange semi-major axes or eccentricities when they approach each other, orbital parameters that are used to describe the relation of co-orbital objects are the longitude of the periapsis difference and the mean longitude difference. The longitude of the periapsis is the sum of the longitude and the mean anomaly and the mean longitude is the sum of the longitude of the ascending node. Trojan objects orbit 60° ahead of or behind a massive object. The best known example are the asteroids that orbit ahead of or behind Jupiter around the Sun, trojan objects do not orbit exactly at one of either Lagrangian points, but do remain relatively close to it, appearing to slowly orbit it. In technical terms, they librate around =, the point around which they librate is the same, irrespective of their mass or orbital eccentricity. There are several thousand known trojan minor planets orbiting the Sun, most of these orbit near Jupiters Lagrangian points, the traditional Jupiter trojans. As of 2015 there are also 13 Neptune trojans,7 Mars trojans,1 Uranus trojan and 1 Earth trojan known to exist, the Saturnian system contains two sets of trojan moons. Both Tethys and Dione have two moons, Telesto and Calypso in Tethyss L4 and L5 respectively, and Helene and Polydeuces in Diones L4. Polydeuces is noticeable for its wide libration, it wanders as far as ±30° from its Lagrangian point and ±2% from its orbital radius. A pair of co-orbital exoplanets was proposed to be orbiting the star Kepler-223, the possibility of a trojan planet to Kepler-91b was studied but the conclusion was that the transit-signal was a false-positive. One possibility for the zone is a trojan planet of a giant planet close to its star. Objects in a horseshoe orbit librate around 180° from the primary and their orbits encompass both equilateral Lagrangian points, i. e. L4 and L5. The Saturnian moons Janus and Epimetheus share their orbits, the difference in semi-major axes being less than eithers mean diameter and this means the moon with the smaller semi-major axis will slowly catch up with the other. As it does this, the moons gravitationally tug at each other, increasing the semi-major axis of the moon that has caught up and this reverses their relative positions proportionally to their masses and causes this process to begin anew with the moons roles reversed
24.
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
25.
Earth trojan
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An Earth trojan is an asteroid that orbits the Sun in the vicinity of the Earth–Sun Lagrangian points L4 and L5, thus having an orbit similar to Earths. Only one Earth trojan has so far been discovered, the name trojan was first used in 1906 for the Jupiter trojans, the asteroids that were observed near the Lagrangian points of Jupiters orbit. Current L42010 TK7, A 300-meter-diameter asteroid, discovered using the Wide-field Infrared Survey Explorer satellite and it is the only confirmed Earth trojan as of 2011–2012. L5 No known objects are thought to be L5 trojans of Earth. 2010 TK7 was discovered using the Wide-field Infrared Survey Explorer satellite, in February 2017, the OSIRIS-REx spacecraft performed a search from within the L4 region on its way to asteroid Bennu. The orbits of any Earth trojans could make them less costly to reach than the Moon. Such asteroids could one day be useful as sources of elements that are rare near Earths surface, on Earth, siderophiles such as iridium are difficult to find, having largely sunk to the core of the planet shortly after its formation. A hypothetical planet-sized Earth trojan the size of Mars, given the name Theia, is thought by proponents of the hypothesis to be the origin of the Moon. The hypothesis states that the Moon formed after Earth and Theia collided head-on and this material eventually accreted around Earth and into a single orbiting body, the Moon. At the same time, material from Theia mixed and combined with Earths mantle, supporters of the giant-impact hypothesis theorise that Earths large core in relation to its overall volume is as a result of this combination. Several other small objects have found on an orbital path associated with Earth. Although these objects are in 1,1 orbital resonance, they are not Earth trojans because they do not librate around a definite Sun–Earth Lagrangian point, Earth has another noted companion, asteroid 3753 Cruithne. About 5 km across, it has a type of orbital resonance called an overlapping horseshoe. 2016 HO3, an asteroid discovered on 27 April 2016, is possibly the most stable quasi-satellite of Earth
26.
Joseph-Louis Lagrange
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Joseph-Louis Lagrange, born Giuseppe Lodovico Lagrangia or Giuseppe Ludovico De la Grange Tournier, was an Italian and French Enlightenment Era mathematician and astronomer. He made significant contributions to the fields of analysis, number theory, in 1787, at age 51, he moved from Berlin to Paris and became a member of the French Academy of Sciences. He remained in France until the end of his life, Lagrange was one of the creators of the calculus of variations, deriving the Euler–Lagrange equations for extrema of functionals. He also extended the method to take into account possible constraints and he proved that every natural number is a sum of four squares. His treatise Theorie des fonctions analytiques laid some of the foundations of group theory, in calculus, Lagrange developed a novel approach to interpolation and Taylor series. Born as Giuseppe Lodovico Lagrangia, Lagrange was of Italian and French descent and his mother was from the countryside of Turin. He was raised as a Roman Catholic, a career as a lawyer was planned out for Lagrange by his father, and certainly Lagrange seems to have accepted this willingly. He studied at the University of Turin and his subject was classical Latin. At first he had no enthusiasm for mathematics, finding Greek geometry rather dull. It was not until he was seventeen that he showed any taste for mathematics – his interest in the subject being first excited by a paper by Edmond Halley which he came across by accident. Alone and unaided he threw himself into mathematical studies, at the end of a years incessant toil he was already an accomplished mathematician, in that capacity, Lagrange was the first to teach calculus in an engineering school. In this Academy one of his students was François Daviet de Foncenex, Lagrange is one of the founders of the calculus of variations. Starting in 1754, he worked on the problem of tautochrone, Lagrange wrote several letters to Leonhard Euler between 1754 and 1756 describing his results. He outlined his δ-algorithm, leading to the Euler–Lagrange equations of variational calculus, Lagrange also applied his ideas to problems of classical mechanics, generalizing the results of Euler and Maupertuis. Euler was very impressed with Lagranges results, Lagrange published his method in two memoirs of the Turin Society in 1762 and 1773. Many of these are elaborate papers, the article concludes with a masterly discussion of echoes, beats, and compound sounds. Other articles in volume are on recurring series, probabilities. The next work he produced was in 1764 on the libration of the Moon, and an explanation as to why the face was always turned to the earth
27.
Three-body problem
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The three-body problem is a special case of the n-body problem. Historically, the first specific three-body problem to receive extended study was the one involving the Moon, the Earth, in an extended modern sense, a three-body problem is a class of problems in classical or quantum mechanics that model the motion of three particles. The gravitational problem of three bodies in its traditional sense dates in substance from 1687, when Isaac Newton published his Principia. The physical problem was addressed by Amerigo Vespucci and subsequently by Galileo Galilei, however the accuracy of the lunar theory was low, due to the perturbing effect of the Sun and planets on the motion of the Moon around the Earth. They submitted their competing first analyses to the Académie Royale des Sciences in 1747 and it was in connection with these researches, in Paris, in the 1740s, that the name three-body problem began to be commonly used. An account published in 1761 by Jean le Rond dAlembert indicates that the name was first used in 1747, in 1887, mathematicians Heinrich Bruns and Henri Poincaré showed that there is no general analytical solution for the three-body problem given by algebraic expressions and integrals. The motion of three bodies is generally non-repeating, except in special cases, a prominent example of the classical three-body problem is the movement of a planet with a satellite around a star. In this case, the problem is simplified to two instances of the two-body problem, however, the effect of the star on the movement of the satellite around the planet can be considered as a perturbation. While a spaceflight involving a gravity assist tends to be at least a problem, once far away from the Earth when Earths gravity becomes negligible. The general statement for the three body problem is as follows, in the circular restricted three-body problem, two massive bodies move in circular orbits around their common center of mass, and the third mass is negligible with respect to the other two. It can be useful to consider the effective potential, in 1767 Leonhard Euler found three families of periodic solutions in which the three masses are collinear at each instant. In 1772 Lagrange found a family of solutions in which the three form an equilateral triangle at each instant. Together, these form the central configurations for the three-body problem. These solutions are valid for any mass ratios, and the move on Keplerian ellipses. These five families are the only solutions for which there are explicit analytic formulae. In 1893 Meissel stated what is called the Pythagorean three-body problem. Burrau further investigated this problem in 1913, in 1967 Victor Szebehely and coworkers established eventual escape for this problem using numerical integration, while at the same time finding a nearby periodic solution. In 1911, United States scientist William Duncan MacMillan found one special solution, in 1961, Russian mathematician Sitnikov improved this solution
28.
Horseshoe orbit
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A horseshoe orbit is a type of co-orbital motion of a small orbiting body relative to a larger orbiting body. The orbital period of the body is very nearly the same as for the larger body. The loop is not closed but will drift forward or backward slightly each time, when the object approaches the larger body closely at either end of its trajectory, its apparent direction changes. Over an entire cycle the center traces the outline of a horseshoe, asteroids in horseshoe orbits with respect to Earth include 54509 YORP,2002 AA29,2010 SO16,2015 SO2 and possibly 2001 GO2. A broader definition includes 3753 Cruithne, which can be said to be in a compound and/or transition orbit, saturns moons Epimetheus and Janus occupy horseshoe orbits with respect to each other. The following explanation relates to an asteroid which is in such an orbit around the Sun, the asteroid is in almost the same solar orbit as Earth. Both take approximately one year to orbit the Sun and it is also necessary to grasp two rules of orbit dynamics, A body closer to the Sun completes an orbit more quickly than a body further away. If a body accelerates along its orbit, its orbit moves outwards from the Sun, if it decelerates, the orbital radius decreases. The horseshoe orbit arises because the attraction of the Earth changes the shape of the elliptical orbit of the asteroid. The shape changes are small but result in significant changes relative to the Earth. The horseshoe becomes apparent only when mapping the movement of the relative to both the Sun and the Earth. The asteroid always orbits the Sun in the same direction, however, it goes through a cycle of catching up with the Earth and falling behind, so that its movement relative to both the Sun and the Earth traces a shape like the outline of a horseshoe. Starting at point A, on the ring between L5 and Earth, the satellite is orbiting faster than the Earth and is on its way toward passing between the Earth and the Sun. But Earths gravity exerts an outward accelerating force, pulling the satellite into an orbit which decreases its angular speed. When the satellite gets to point B, it is traveling at the speed as Earth. Earths gravity is still accelerating the satellite along the orbital path, eventually, at Point C, the satellite reaches a high and slow enough orbit such that it starts to lag behind Earth. It then spends the next century or more appearing to drift backwards around the orbit when viewed relative to the Earth and its orbit around the Sun still takes only slightly more than one Earth year. Given enough time, the Earth and the satellite will be on opposite sides of the Sun, eventually the satellite comes around to point D where Earths gravity is now reducing the satellites orbital velocity
29.
Edward Emerson Barnard
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Edward Emerson Barnard was an American astronomer. He was commonly known as E. E. Barnard, and was recognized as an observational astronomer. He is best known for his discovery of the proper motion of Barnards Star in 1916. Barnard was born in Nashville, Tennessee, to Reuben Barnard and Elizabeth Jane Barnard and his father died three months before his birth, so he grew up in an impoverished family and did not receive much in the way of formal education. His first interest was in the field of photography, and he became an assistant at the age of nine. He later developed an interest in astronomy, in 1876 he purchased a 5-inch refractor telescope, and in 1881 he discovered his first comet, but failed to announce his discovery. He found his second later the same year and a third in 1882. While he was working at a photography studio he was married to the English-born woman Rhoda Calvert in 1881. In the 1880s, Hulbert Harrington Warner offered US$200 per discovery of a new comet, Edward discovered a total of five, and used the money to build a house for himself and his bride. With his name being brought to the attention of astronomers in Nashville. Barnard never graduated from the school, but he did receive the honorary degree Vanderbilt has ever awarded. In 1889 he observed the moon Iapetus pass behind Saturns rings, as he watched Iapetus pass through the space between Saturns innermost rings and the planet itself, he saw a shadow pass over the moon. Although he did not realise it at the time, he had discovered proof of the spokes of Saturn and these spokes were doubted at first, but confirmed by the spacecraft Voyager 1. In 1892 he made observations of a nova and was the first to notice the gaseous emissions, the same year he also discovered Amalthea, the fifth moon of Jupiter. He was the first to discover a new moon of Jupiter since Galileo Galilei in 1609 and this was the last satellite discovered by visual observation. In 1895 he joined the University of Chicago as professor of astronomy, there he was able to use the 40-inch telescope at Yerkes Observatory. Much of his work during this period was taking photographs of the Milky Way, together with Max Wolf, he discovered that certain dark regions of the galaxy were actually clouds of gas and dust that obscured the more distant stars in the background. From 1905, his niece Mary R. Calvert worked as his assistant, the faint Barnards Star is named for Edward Barnard after he discovered in 1916 that it had a very large proper motion, relative to other stars
30.
Moons of Saturn
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The moons of Saturn are numerous and diverse, ranging from tiny moonlets less than 1 kilometer across to the enormous Titan, which is larger than the planet Mercury. Saturn has 62 moons with confirmed orbits,53 of which have names and only 13 of which have larger than 50 kilometers. Seven Saturnian moons are large enough to be ellipsoidal in shape, twenty-four of Saturns moons are regular satellites, they have prograde orbits not greatly inclined to Saturns equatorial plane. They include the seven satellites, four small moons that exist in a trojan orbit with larger moons. Two other known regular satellites orbit within gaps in Saturns rings, the relatively large Hyperion is locked in a resonance with Titan. The remaining regular moons orbit near the edge of the A Ring, within G Ring. The regular satellites are traditionally named after Titans and Titanesses or other associated with the mythological Saturn. The remaining 38, all small except one, are irregular satellites, whose orbits are farther from Saturn, have high inclinations. These moons are probably captured minor planets, or debris from the breakup of such bodies after they were captured, the irregular satellites have been classified by their orbital characteristics into the Inuit, Norse, and Gallic groups, and their names are chosen from the corresponding mythologies. The largest of the moons is Phoebe, the ninth moon of Saturn. The rings of Saturn are made up of objects ranging in size from microscopic to moonlets hundreds of meters across, before the advent of telescopic photography, eight moons of Saturn were discovered by direct observation using optical telescopes. Saturns largest moon, Titan, was discovered in 1655 by Christiaan Huygens using a 57-millimeter objective lens on a telescope of his own design. Tethys, Dione, Rhea and Iapetus were discovered between 1671 and 1684 by Giovanni Domenico Cassini, Mimas and Enceladus were discovered in 1789 by William Herschel. Hyperion was discovered in 1848 by W. C, the use of long-exposure photographic plates made possible the discovery of additional moons. The first to be discovered in this manner, Phoebe, was found in 1899 by W. H. Pickering, in 1966 the tenth satellite of Saturn was discovered by Audouin Dollfus, when the rings were observed edge-on near an equinox. A few years later it was realized that all observations of 1966 could only be explained if another satellite had been present and this object is now known as Epimetheus, the eleventh moon of Saturn. It shares the orbit with Janus—the only known example of co-orbitals in the Solar System. In 1980 three additional Saturnian moons were discovered from the ground and later confirmed by the Voyager probes and they are trojan moons Dione, Tethys
31.
Phoebe (moon)
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Phoebe is an irregular satellite of Saturn with a mean diameter of 213 km. It was the first satellite to be discovered photographically, Phoebe was the first target encountered upon the arrival of the Cassini spacecraft in the Saturn system in 2004, and is thus unusually well-studied for an irregular satellite of its size. Cassinis trajectory to Saturn and time of arrival were specifically chosen to permit this flyby, after the encounter and its insertion into orbit, Cassini did not go much beyond the orbit of Iapetus. Phoebe is roughly spherical and has a differentiated interior and it was spherical and hot early in its history and was battered out of roundness by repeated impacts. It is thought to be a captured planetesimal from the Kuiper belt and it was the first satellite to be discovered photographically. Phoebe was named after Phoebe, a Titan in Greek mythology that was associated with the Moon and it is also designated Saturn IX in some scientific literature. The IAU nomenclature standards have stated that features on Phoebe are to be named characters in the Greek myth of Jason. In 2005, the IAU officially named 24 craters, Phoebes orbit is retrograde, i. e. it orbits Saturn opposite to Saturns rotation. For more than 100 years, Phoebe was Saturns outermost known moon, Phoebe is almost 4 times more distant from Saturn than its nearest major neighbor, and is substantially larger than any of the other moons orbiting planets at comparable distances. All of Saturns regular moons except Iapetus orbit very nearly in the plane of Saturns equator, the outer irregular satellites follow moderately to highly eccentric orbits, and none are expected to rotate synchronously as all the inner moons of Saturn do. The Phoebe ring is one of the rings of Saturn and this ring is tilted 27 degrees from Saturns equatorial plane. It extends from at least 128 to 207 times the radius of Saturn, the ring is about 20 times as thick as the diameter of the planet. Inwardly migrating ring material would thus strike Iapetuss leading hemisphere, created its two-tone coloration, although very large, the ring is virtually invisible—it was discovered using NASAs infrared Spitzer Space Telescope. Phoebe is roughly spherical and has a diameter of 213±1.4 km, Phoebe rotates every nine hours 16 minutes and it completes a full orbit around Saturn in about 18 months. Its surface temperature is 75 K, most of Saturns inner moons have very bright surfaces, but Phoebes albedo is very low, as dark as lampblack. The Phoebean surface is heavily scarred, with craters up to 80 kilometres across. Phoebes dark coloring initially led to scientists surmising that it was a captured asteroid and these are chemically very primitive and are thought to be composed of original solids that condensed out of the solar nebula with little modification since then. In addition, quantities of carbon dioxide have been detected on the surface and it is estimated that Phoebe is about 50% rock, as opposed to the 35% or so that typifies Saturns inner moons
32.
Minute and second of arc
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A minute of arc, arcminute, arc minute, or minute arc is a unit of angular measurement equal to 1/60 of one degree. Since one degree is 1/360 of a turn, one minute of arc is 1/21600 of a turn, a second of arc, arcsecond, or arc second is 1/60 of an arcminute, 1/3600 of a degree, 1/1296000 of a turn, and π/648000 of a radian. To express even smaller angles, standard SI prefixes can be employed, the number of square arcminutes in a complete sphere is 4 π2 =466560000 π ≈148510660 square arcminutes. The standard symbol for marking the arcminute is the prime, though a single quote is used where only ASCII characters are permitted. One arcminute is thus written 1′ and it is also abbreviated as arcmin or amin or, less commonly, the prime with a circumflex over it. The standard symbol for the arcsecond is the prime, though a double quote is commonly used where only ASCII characters are permitted. One arcsecond is thus written 1″ and it is also abbreviated as arcsec or asec. In celestial navigation, seconds of arc are used in calculations. This notation has been carried over into marine GPS receivers, which normally display latitude and longitude in the format by default. An arcsecond is approximately the angle subtended by a U. S. dime coin at a distance of 4 kilometres, a milliarcsecond is about the size of a dime atop the Eiffel Tower as seen from New York City. A microarcsecond is about the size of a period at the end of a sentence in the Apollo mission manuals left on the Moon as seen from Earth, since antiquity the arcminute and arcsecond have been used in astronomy. The principal exception is Right ascension in equatorial coordinates, which is measured in units of hours, minutes. These small angles may also be written in milliarcseconds, or thousandths of an arcsecond, the unit of distance, the parsec, named from the parallax of one arcsecond, was developed for such parallax measurements. It is the distance at which the radius of the Earths orbit would subtend an angle of one arcsecond. The ESA astrometric space probe Gaia is hoped to measure star positions to 20 microarcseconds when it begins producing catalog positions sometime after 2016, there are about 1.3 trillion µas in a turn. Currently the best catalog positions of stars actually measured are in terms of milliarcseconds, apart from the Sun, the star with the largest angular diameter from Earth is R Doradus, a red supergiant with a diameter of 0.05 arcsecond. The dwarf planet Pluto has proven difficult to resolve because its angular diameter is about 0.1 arcsecond, space telescopes are not affected by the Earths atmosphere but are diffraction limited. For example, the Hubble space telescope can reach a size of stars down to about 0. 1″
33.
624 Hektor
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624 Hektor is the largest Jupiter trojan. It was discovered in 1907 by August Kopff, Hektor is a D-type asteroid, dark and reddish in colour. It lies in Jupiters leading Lagrangian point, L4, called the Greek node after one of the two sides in the legendary Trojan War, Hektor is named after the Trojan hero Hektor and is thus one of two trojan asteroids that is misplaced in the wrong camp. Hektor is one of the most elongated bodies of its size in the Solar System and it is thought that Hektor might be a contact binary like 216 Kleopatra. Hubble Space Telescope observations of Hektor in 1993 did not show an obvious bilobate shape because of an angular resolution. On 17 July 2006, the Keck 10-meter-II-telescope and its laser guide star adaptive optics system indicated a bilobate shape for Hektor. Additionally, a 12-km-diameter moon of Hektor, named Skamandrios, S/20061, was detected orbiting with an axis of 623.5 km. It was confirmed with Keck observations in November 2011, Hektor is, so far, the only known binary trojan asteroid in the L4 point and the first known trojan with a satellite companion. 617 Patroclus, another large trojan asteroid located in the L5, is composed of two same-sized components
34.
Johann Palisa
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Johann Palisa was an Austrian astronomer, born in Troppau in Austrian Silesia. He was a discoverer of asteroids, discovering 122 in all. Some of his discoveries include 153 Hilda,216 Kleopatra,243 Ida,253 Mathilde,324 Bamberga. He was awarded the Valz Prize from the French Academy of Sciences in 1906, the Phocaea main-belt asteroid 914 Palisana, discovered by Max Wolf in 1919, and the lunar crater Palisa were named in his honour. From 1866 to 1870, Palisa studied mathematics and astronomy at the University of Vienna, however, despite this, by 1870 he was an assistant at the Universitys observatory, and a year later gained a position at the observatory in Geneva. A few years later, in 1872, at the age of 24, while at Pula, he discovered his first asteroid,136 Austria, on March 18,1874. Along with this, he discovered twenty-seven minor planets and one comet, during his stay in Pula he used a small six-inch refractor telescope to aid in his research. Between 1874 and 1923 Palisa discovered 122 asteroids ranging from 136 Austria to 1073 Gellivara and he worked from Pola and Vienna. He also discovered the parabolic comet C/1879 Q1 in the year 1879, one of his discoveries was 253 Mathilde, which was visited by the spacecraft NEAR Shoemaker on June 27,1997. The robotic probe passed within 1200 km of Mathilde at 12,56 UT at 9.93 km/s, returning imaging and other instrument data including over 500 images which covered 60% of Mathildes surface. Portraits of Johann Palisa from the Lick Observatory Records Digital Archive, UC Santa Cruz Librarys Digital Collections von Hepperger, J. Anzeige des Todes von Johann Palisa
35.
Vienna
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Vienna is the capital and largest city of Austria and one of the nine states of Austria. Vienna is Austrias primary city, with a population of about 1.8 million, and its cultural, economic and it is the 7th-largest city by population within city limits in the European Union. Today, it has the second largest number of German speakers after Berlin, Vienna is host to many major international organizations, including the United Nations and OPEC. The city is located in the part of Austria and is close to the borders of the Czech Republic, Slovakia. These regions work together in a European Centrope border region, along with nearby Bratislava, Vienna forms a metropolitan region with 3 million inhabitants. In 2001, the city centre was designated a UNESCO World Heritage Site, apart from being regarded as the City of Music because of its musical legacy, Vienna is also said to be The City of Dreams because it was home to the worlds first psycho-analyst – Sigmund Freud. The citys roots lie in early Celtic and Roman settlements that transformed into a Medieval and Baroque city and it is well known for having played an essential role as a leading European music centre, from the great age of Viennese Classicism through the early part of the 20th century. The historic centre of Vienna is rich in architectural ensembles, including Baroque castles and gardens, Vienna is known for its high quality of life. In a 2005 study of 127 world cities, the Economist Intelligence Unit ranked the city first for the worlds most liveable cities, between 2011 and 2015, Vienna was ranked second, behind Melbourne, Australia. Monocles 2015 Quality of Life Survey ranked Vienna second on a list of the top 25 cities in the world to make a base within, the UN-Habitat has classified Vienna as being the most prosperous city in the world in 2012/2013. Vienna regularly hosts urban planning conferences and is used as a case study by urban planners. Between 2005 and 2010, Vienna was the worlds number-one destination for international congresses and it attracts over 3.7 million tourists a year. The English name Vienna is borrowed from the homonymous Italian version of the name or the French Vienne. The etymology of the name is still subject to scholarly dispute. Some claim that the name comes from Vedunia, meaning forest stream, which produced the Old High German Uuenia. A variant of this Celtic name could be preserved in the Czech and Slovak names of the city, the name of the city in Hungarian, Serbo-Croatian and Ottoman Turkish has a different, probably Slavonic origin, and originally referred to an Avar fort in the area. Slovene-speakers call the city Dunaj, which in other Central European Slavic languages means the Danube River, evidence has been found of continuous habitation since 500 BC, when the site of Vienna on the Danube River was settled by the Celts. In 15 BC, the Romans fortified the city they called Vindobona to guard the empire against Germanic tribes to the north
36.
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
37.
Achilles
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In Greek mythology, Achilles was a Greek hero of the Trojan War and the central character and greatest warrior of Homers Iliad. His mother was the immortal nymph Thetis, and his father, Achilles’ most notable feat during the Trojan War was the slaying of the Trojan hero Hector outside the gates of Troy. Although the death of Achilles is not presented in the Iliad, other sources concur that he was killed near the end of the Trojan War by Paris, later legends state that Achilles was invulnerable in all of his body except for his heel. Alluding to these legends, the term Achilles heel has come to mean a point of weakness, Achilles name can be analyzed as a combination of ἄχος grief and λαός a people, tribe, nation. In other words, Achilles is an embodiment of the grief of the people, Achilles role as the hero of grief forms an ironic juxtaposition with the conventional view of Achilles as the hero of κλέος kleos. Laos has been construed by Gregory Nagy, following Leonard Palmer, to mean a corps of soldiers, a muster. With this derivation, the name would have a meaning in the poem, when the hero is functioning rightly, his men bring grief to the enemy. The poem is in part about the misdirection of anger on the part of leadership, R. S. P. Beekes has suggested a Pre-Greek origin of the name. The name Achilleus was a common and attested name among the Greeks soon after the 7th century BC. It was also turned into the female form Ἀχιλλεία attested in Attica in the 4th century BC and, in the form Achillia, Achilles was the son of the Nereid Thetis and Peleus, the king of the Myrmidons. Zeus and Poseidon had been rivals for the hand of Thetis until Prometheus, for this reason, the two gods withdrew their pursuit, and had her wed Peleus. Thetis, although a daughter of the sea-god Nereus, was brought up by Hera. According to the Achilleid, written by Statius in the 1st century AD, and to no surviving previous sources, however, he was left vulnerable at the part of the body by which she held him, his heel. It is not clear if this version of events was known earlier, in another version of this story, Thetis anointed the boy in ambrosia and put him on top of a fire, to burn away the mortal parts of his body. She was interrupted by Peleus and abandoned both father and son in a rage, however, none of the sources before Statius makes any reference to this general invulnerability. To the contrary, in the Iliad Homer mentions Achilles being wounded, in Book 21 the Paeonian hero Asteropaeus, son of Pelagon and he cast two spears at once, one grazed Achilles elbow, drawing a spurt of blood. Peleus entrusted Achilles to Chiron the Centaur, on Mt. Pelion, Achilles consuming rage is at times wavering, but at other times he cannot be cooled. Thetis foretold that her sons fate was either to gain glory and die young, or to live a long, Achilles chose the former, and decided to take part in the Trojan war
38.
Troy
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The present-day location is known as Hisarlik. It was the setting of the Trojan War described in the Greek Epic Cycle, in particular in the Iliad, a new capital called Ilium was founded on the site in the reign of the Roman Emperor Augustus. It flourished until the establishment of Constantinople and declined gradually in the Byzantine era and these excavations revealed several cities built in succession. Troy VII has been identified with the city that the Hittites called Wilusa, the origin of the Greek Ἴλιον. Today, the hill at Hisarlık has given its name to a village near the ruins. It lies within the province of Çanakkale, some 30 km south-west of the provincial capital, the map here shows the adapted Scamander estuary with Ilium a little way inland across the Homeric plain. Troy was added to the UNESCO World Heritage list in 1998, Ancient Greek historians variously placed the Trojan War in the 12th, 13th, or 14th centuries BC, Eratosthenes to 1184 BC, Herodotus to 1250 BC, Duris of Samos to 1334 BC. Modern archaeologists associate Homeric Troy with archaeological Troy VII, in the Iliad, the Achaeans set up their camp near the mouth of the River Scamander, where they had beached their ships. The city of Troy itself stood on a hill, across the plain of Scamander, recent geological findings have permitted the identification of the ancient Trojan coastline, and the results largely confirm the accuracy of the Homeric geography of Troy. In November 2001, the geologist John C, kraft from the University of Delaware and the classicist John V. Luce from Trinity College, Dublin, presented the results of investigations, begun in 1977, into the geology of the region. Besides the Iliad, there are references to Troy in the major work attributed to Homer. The Homeric legend of Troy was elaborated by the Roman poet Virgil in his Aeneid, the Greeks and Romans took for a fact the historicity of the Trojan War and the identity of Homeric Troy with the site in Anatolia. Alexander the Great, for example, visited the site in 334 BC and there made sacrifices at tombs associated with the Homeric heroes Achilles and Patroclus. After the 1995 find of a Luwian biconvex seal at Troy VII, with the rise of critical history, Troy and the Trojan War were, for a long time, consigned to the realms of legend. However, the location of ancient Troy had from classical times remained the subject of interest. The Troad peninsula was anticipated to be the location, leChavaliers location, published in his Voyage de la Troade, was the most commonly accepted theory for almost a century. In 1822, the Scottish journalist Charles Maclaren was the first to identify with confidence the position of the city as it is now known, the hill, near the city of Çanakkale, was known as Hisarlık. In 1868, Heinrich Schliemann visited Calvert and secured permission to excavate Hisarlık, in 1871–73 and 1878–79, he excavated the hill and discovered the ruins of a series of ancient cities dating from the Bronze Age to the Roman period
39.
Patroclus
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In Greek mythology, as recorded in Homers Iliad, Patroclus was the son of Menoetius, grandson of Actor, King of Opus, and Achilles beloved and brother-in-arms. According to Hyginus, Patroclus is the child of Menoetius and Philomela, Homer also references Menoetius as the individual who gave Patroclus to Peleus. Menoetius is the son of Actor, King of Opus in Locris by Aegina, Aegina was a daughter of Asopus and mother of Aeacus by Zeus. Aeacus was father of Peleus, Telamon and Phocus, Actor was a son of Deioneus, King of Phocis and Diomede. His paternal grandparents were Aeolus of Thessaly and Enarete and his maternal grandparents were Xuthus and Creusa, daughter of Erechtheus and Praxithea. During his childhood, Patroclus had killed another child in anger over a game, Menoetius gave Patroclus to Peleus, Achilles father, who named Patroclus one of Achilles henchmen as Patroclus and Achilles grew up together. Patroclus acted as a role model for Achilles, as he was both older than Achilles and wise regarding counsel. According to the Iliad, when the tide of war had turned against the Greeks, Achilles consented, giving Patroclus the armor Achilles had received from his father, in order for Patroclus to impersonate Achilles. Achilles then told Patroclus to return after beating the Trojans back from their ships, Patroclus defied Achilles order and pursued the Trojans back to the gates of Troy. Patroclus killed many Trojans, including a son of Zeus, Sarpedon, while battling, Patroclus wits were removed by Apollo, after which Patroclus was hit with the spear of Euphorbos. Hector then killed Patroclus by stabbing him in the stomach with a spear, Achilles retrieved the body, which had been stripped of armor by Hector and protected on the battlefield by Menelaus and Ajax. Achilles did not allow for the burial of Patroclus body until the ghost of Patroclus appeared and demanded burial in order to pass into Hades, Patroclus was then cremated on a funeral pyre, which was covered in the hair of his sorrowful companions. As the cutting of hair was a sign of grief while also acting as a sign of the separation of the living and the dead, the ashes of Achilles were said to have been buried in a golden urn along with those of Patroclus by the Hellespont. Although Homer does not mention it, there is whether or not Achilles. Morales and Mariscal continue stating, there is a tradition concerning the nature of the relationship between the two heroes. According to Grace Ledbetter, there is a train of thought that Patroclus could have been a representation of the side of Achilles. In fact, the first line of Homers Iliad mentions Achilles anger, Ledbetter does so by comparing how Thetis comforts the weeping Achilles in Book 1 of the Iliad to how Achilles comforts Patroclus as he weeps in Book 16. Achilles uses a simile containing a young girl looking at her mother to complete the comparison
40.
Hector
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In Greek mythology and Roman Mythology, Hector was a Trojan prince and the greatest fighter for Troy in the Trojan War. As the first-born son of King Priam and Queen Hecuba, who was a descendant of Dardanus and Tros, the founder of Troy, he was a prince of the royal house and he was married to Andromache, with whom he had an infant son, Scamandrius. He acted as leader of the Trojans and their allies in the defense of Troy, during the European Middle Ages, Hector figures as one of the Nine Worthies noted by Jacques de Longuyon, known not only for his courage but also for his noble and courtly nature. Indeed, Homer places Hector as peace-loving, thoughtful as well as bold, a son, husband and father. James Redfield writes of Hector as a martyr to loyalties, a witness to the things of this world, in Greek, Héktōr is a derivative of the verb ἔχειν ékhein, archaic form *ἕχειν hékhein, to have or to hold from Proto-Indo-European *seǵh- to hold. Héktōr, or Éktōr as found in Aeolic poetry, is also an epithet of Zeus in his capacity as he who holds, Hectors name could thus be taken to mean holding fast. According to the Iliad, Hector does not approve of war between the Greeks and the Trojans, for ten years, the Achaeans besieged Troy and their allies in the east. Hector commanded the Trojan army, with a number of subordinates including Polydamas, and his brothers Deiphobus, Helenus, and Paris. By all accounts, Hector was the best warrior the Trojans and all their allies could field, diomedes and Odysseus, when faced with his attack, described him as what Robert Fagles translated as an incredible dynamite, and a maniac. In the Iliad, Hectors exploits in the war prior to the events of the book are recapitulated and he had fought the Greek champion Protesilaus in single combat at the start of the war and killed him. A prophecy had stated that the first Greek to land on Trojan soil would die, thus, Protesilaus, Ajax, and Odysseus would not land. Finally, Odysseus threw his shield out and landed on that, in the ensuing fight, Hector killed him, fulfilling the prophecy. The Argives were initially reluctant to accept the challenge, however, after Nestors chiding, nine Greek heroes stepped up to the challenge and drew by lot to see who was to face Hector. Ajax wins and fights Hector to a stalemate for the entire day, with neither able to achieve victory, they express admiration for each others courage, skill, and strength. Hector gave Ajax his sword, which Ajax later uses to kill himself, Ajax gives Hector his girdle, which later was used to attach Hectors corpse to Achilles chariot by which he is dragged around the walls of Troy. The Greek and the Trojans make a truce to bury the dead, in the early dawn the next day the Greeks take advantage of it to build a wall and ditch around the ships. Zeus is watching in a distance, another mention of Hectors exploits in the early years of war was given in the Iliad book 9. During the embassy to Achilles, Odysseus, Phoenix and Ajax all try to persuade Achilles to rejoin the fight and he then claims, There he stood up to me alone one day, and he barely escaped my onslaught
41.
Gravitational potential
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It is analogous to the electric potential with mass playing the role of charge. The reference location, where the potential is zero, is by convention infinitely far away from any mass, in mathematics, the gravitational potential is also known as the Newtonian potential and is fundamental in the study of potential theory. The gravitational potential is the potential energy per unit mass, U = m V. Potential energy is equal to the work done by the field moving a body to its given position in space from infinity. If the body has a mass of 1 unit, then the energy to be assigned to that body is equal to the gravitational potential. So the potential can be interpreted as the negative of the work done by the field moving a unit mass in from infinity. In some situations, the equations can be simplified by assuming a field that is independent of position. For instance, in a close to the surface of the Earth. In that case, the difference in energy from one height to another is, to a good approximation, linearly related to the difference in height. The potential has units of energy per mass, e. g. J/kg in the MKS system. By convention, it is always negative where it is defined, the gravitational field, and thus the acceleration of a small body in the space around the massive object, is the negative gradient of the gravitational potential. Thus the negative of a gradient yields positive acceleration toward a massive object. The magnitude of the acceleration therefore follows a square law. The potential associated with a distribution is the superposition of the potentials of point masses. If the mass distribution is a collection of point masses. Mn, then the potential of the distribution at the point x is V = ∑ i =1 n − G m i | x − x i |. If the mass distribution is given as a mass measure dm on three-dimensional Euclidean space R3, then the potential is the convolution of −G/|r| with dm. In good cases this equals the integral V = − ∫ R3 G | x − r | d m, where |x − r| is the distance between the points x and r
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Albedo
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Albedo is a measure for reflectance or optical brightness. It is dimensionless and measured on a scale from zero to one, surface albedo is defined as the ratio of radiation reflected to the radiation incident on a surface. The proportion reflected is not only determined by properties of the surface itself and these factors vary with atmospheric composition, geographic location and time. While bi-hemispherical reflectance is calculated for an angle of incidence. The temporal resolution may range from seconds to daily, seasonal or annual averages, unless given for a specific wavelength, albedo refers to the entire spectrum of solar radiation. Due to measurement constraints, it is given for the spectrum in which most solar energy reaches the surface. This spectrum includes visible light, which explains why surfaces with a low albedo appear dark, albedo is an important concept in climatology, astronomy, and environmental management. The term albedo was introduced into optics by Johann Heinrich Lambert in his 1760 work Photometria, any albedo in visible light falls within a range of about 0.9 for fresh snow to about 0.04 for charcoal, one of the darkest substances. Deeply shadowed cavities can achieve an effective albedo approaching the zero of a black body, when seen from a distance, the ocean surface has a low albedo, as do most forests, whereas desert areas have some of the highest albedos among landforms. Most land areas are in a range of 0.1 to 0.4. The average albedo of Earth is about 0.3 and this is far higher than for the ocean primarily because of the contribution of clouds. Earths surface albedo is regularly estimated via Earth observation satellite sensors such as NASAs MODIS instruments on board the Terra, thereby, the BRDF allows to translate observations of reflectance into albedo. Earths average surface temperature due to its albedo and the effect is currently about 15 °C. If Earth were frozen entirely, the temperature of the planet would drop below −40 °C. If only the land masses became covered by glaciers, the mean temperature of the planet would drop to about 0 °C. In contrast, if the entire Earth was covered by water — a so-called aquaplanet — the average temperature on the planet would rise to almost 27 °C, hence, the actual albedo α can then be given as, α = α ¯ + D α ¯ ¯. Directional-hemispherical reflectance is sometimes referred to as black-sky albedo and bi-hemispherical reflectance as white-sky albedo and these terms are important because they allow the albedo to be calculated for any given illumination conditions from a knowledge of the intrinsic properties of the surface. The albedos of planets, satellites and asteroids can be used to infer much about their properties, the study of albedos, their dependence on wavelength, lighting angle, and variation in time comprises a major part of the astronomical field of photometry
. Bibcode:2000AJ....120.1140J. doi:10.1086/301453.
