1.
Astronomical unit
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The astronomical unit is a unit of length, roughly the distance from Earth to the Sun. However, that varies as Earth orbits the Sun, from a maximum to a minimum. Originally conceived as the average of Earths aphelion and perihelion, it is now defined as exactly 149597870700 metres, the astronomical unit is used primarily as a convenient yardstick for measuring distances within the Solar System or around other stars. However, it is also a component in the definition of another unit of astronomical length. A variety of symbols and abbreviations have been in use for the astronomical unit. In a 1976 resolution, the International Astronomical Union used the symbol A for the astronomical unit, in 2006, the International Bureau of Weights and Measures recommended ua as the symbol for the unit. In 2012, the IAU, noting that various symbols are presently in use for the astronomical unit, in the 2014 revision of the SI Brochure, the BIPM used the unit symbol au. In ISO 80000-3, the symbol of the unit is ua. Earths orbit around the Sun is an ellipse, the semi-major axis of this ellipse is defined to be half of the straight line segment that joins the aphelion and perihelion. The centre of the sun lies on this line segment. In addition, it mapped out exactly the largest straight-line distance that Earth traverses over the course of a year, knowing Earths shift and a stars shift enabled the stars distance to be calculated. But all measurements are subject to some degree of error or uncertainty, improvements in precision have always been a key to improving astronomical understanding. Improving measurements were continually checked and cross-checked by means of our understanding of the laws of celestial mechanics, the expected positions and distances of objects at an established time are calculated from these laws, and assembled into a collection of data called an ephemeris. NASAs Jet Propulsion Laboratory provides one of several ephemeris computation services, in 1976, in order to establish a yet more precise measure for the astronomical unit, the IAU formally adopted a new definition. Equivalently, by definition, one AU is the radius of an unperturbed circular Newtonian orbit about the sun of a particle having infinitesimal mass. As with all measurements, these rely on measuring the time taken for photons to be reflected from an object. However, for precision the calculations require adjustment for such as the motions of the probe. In addition, the measurement of the time itself must be translated to a scale that accounts for relativistic time dilation

2.
Carolyn S. Shoemaker
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Carolyn Jean Spellmann Shoemaker is an American astronomer and is a co-discoverer of Comet Shoemaker–Levy 9. She once held the record for most comets discovered by an individual, Carolyn Jean Spellmann was born in Gallup, New Mexico, United States. Her family moved to Chico, California, where she and her brother Richard grew up with their parents, Leonard Spellmann, on August 18,1951, she married Gene Shoemaker, a planetary scientist. She gave birth to three children, Christy, Linda, and Patrick Shoemaker, the first job Shoemaker held was at a local school teaching the seventh grade. After not feeling satisfied with her there, she quit to marry. She concentrated her work on searching for comets and planet-crossing asteroids, teamed with astronomer David H. Levy, the Shoemakers identified Shoemaker-Levy 9, a fragmented comet orbiting the planet Jupiter on March 24,1993. After Genes death in 1997, Shoemaker continued to work at the Lowell Observatory with Levy, as of 2002, Shoemaker had discovered 32 comets and over 800 asteroids. She and her husband were awarded the James Craig Watson Medal by the U. S. National Academy of Sciences in 1998, Shoemaker also received the Rittenhouse Medal of the Rittenhouse Astronomical Society in 1988 and the Scientist of the Year Award in 1995. Carolyn Shoemaker is credited by the Minor Planet Center with the discovery of 376 numbered minor planets made between 1980 and 1994, Biography for Carolyn S. Shoemaker at the Internet Movie Database Universe Today page about Carolyn Shoemaker Biography of Carolyn Shoemaker at the USGS Astrogeology Science Center

3.
Eugene Merle Shoemaker
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Eugene Merle Shoemaker, also known as Gene Shoemaker, was an American geologist and one of the founders of the field of planetary science. He is best known for co-discovering the Comet Shoemaker–Levy 9 with his wife Carolyn S. Shoemaker and David H. Levy. Shoemaker was born in Los Angeles, California, the son of Muriel May, a teacher, and George Estel Shoemaker, who worked in farming, business, teaching, and motion pictures. For his Ph. D. degree at Princeton, Shoemaker studied the dynamics of Barringer Meteor Crater, located near Winslow. To understand the dynamics, Shoemaker inspected craters that remained after underground atomic bomb tests at the Nevada Test Site at Yucca Flat and he found a ring of ejected material that included shocked quartz, a form of quartz that has a microscopically unique structure caused by intense pressure. Shoemaker helped pioneer the field of astrogeology by founding the Astrogeology Research Program of the United States Geological Survey in 1961 at Flagstaff, Arizona and he was its first director. He was prominently involved in the Lunar Ranger missions to the Moon, Shoemaker was also involved in the training of the American astronauts. Shoemaker would train astronauts during field trips to Meteor Crater and Sunset Crater near Flagstaff and he was a CBS News television commentator on the early Apollo missions, especially the Apollo 8 and Apollo 11 missions, appearing with Walter Cronkite during live coverage of those flights. He was awarded the John Price Wetherill Medal from the Franklin Institute in 1965, coming to Caltech in 1969, he started a systematic search for Earth orbit-crossing asteroids, which resulted in the discovery of several families of such asteroids, including the Apollo asteroids. Shoemaker advanced the idea that sudden geologic changes can arise from asteroid strikes, previously, astroblemes were thought to be remnants of extinct volcanoes — even on the Moon. Shoemaker received the Barringer Medal in 1984 and a National Medal of Science in 1992, in 1993, he co-discovered Comet Shoemaker–Levy 9 using the 18 Schmidt camera at Palomar Observatory. This comet was unique in that it provided the first opportunity for scientists to observe the impact of a comet. Shoemaker–Levy 9 collided with Jupiter in 1994, the resulting impact caused a massive scar on the face of Jupiter. Most scientists at the time were dubious of whether there would even be any evident markings on the planet, Shoemaker spent much of his later years searching for and finding several previously unnoticed or undiscovered impact craters around the world. Shoemaker died on July 18,1997 during one such expedition in a head on car accident while on the Tanami Road northwest of Alice Springs and his vehicle and another were thought to be using the center, relatively smooth part of a heavily rutted, unimproved road. On seeing Shoemaker approaching, the driver of the vehicle pulled hard to his left, and had Shoemaker done the same. But Shoemaker, as an American accustomed to driving on the side of the road, reflexively pulled hard to his right. A head-on collision in which Shoemakers vehicle was traveling at 80 km/h, the occupants of the other vehicle were rendered unconscious, but not seriously injured

4.
David H. Levy
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David H. Levy is a Canadian astronomer, science writer and discoverer of comets and minor planets, who co-discovered Comet Shoemaker–Levy 9 in 1993, which collided with the planet Jupiter in 1994. Levy was born in Montreal, Quebec, Canada, in 1948 and he developed an interest in astronomy at an early age. However, he pursued and received bachelors and masters degrees in English literature, Levy went on to discover 22 comets, either independently or with Gene and Carolyn Shoemaker. He has written 34 books, mostly on subjects, such as The Quest for Comets, a biography of Pluto-discoverer Clyde Tombaugh in 2006. He has provided periodic articles for Sky and Telescope magazine, as well as Parade Magazine, Sky News and, most recently, periodic comets that Levy co-discovered include 118P/Shoemaker–Levy, 129P/Shoemaker–Levy, 135P/Shoemaker–Levy, 137P/Shoemaker–Levy, 138P/Shoemaker–Levy, 145P/Shoemaker–Levy, and 181P/Shoemaker–Levy. In addition, Levy is the discoverer of two periodic comets, 255P/Levy and P/1991 L3. On February 28,2010, Levy was awarded a Ph. D and he lives in Vail, Arizona and is married to Wendee Levy. Levy and his wife hosted an internet radio talk show on astronomy. Show archives are available in WMA and MP3 formats. Levy is President of the National Sharing the Sky Foundation, Levy is a Master of Astronomy with DeTao Masters Academy. The main-asteroid 3673 Levy was named in his honour, chant Medal of the Royal Astronomical Society of Canada in 1980. Levy was recipient of the 1990 G. Bruce Blair Medal, in 1993 he won the Amateur Achievement Award of the Astronomical Society of the Pacific. In 2007, Levy received the Smithsonian Astrophysical Observatorys Edgar Wilson Award for the discovery of comets, in 2008, a special edition telescope, The Comet Hunter was co-designed by Levy. Holt, June 1990 Established the cataclysmically recurring nature of 1215-17 TV Corvi, August 1987 Carolyn Shoemaker Eugene Shoemaker List of minor planet discoverers David Levys Home Page

5.
Apsis
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An apsis is an extreme point in an objects orbit. The word comes via Latin from Greek and is cognate with apse, for elliptic orbits about a larger body, there are two apsides, named with the prefixes peri- and ap-, or apo- added to a reference to the thing being orbited. For a body orbiting the Sun, the point of least distance is the perihelion, the terms become periastron and apastron when discussing orbits around other stars. For any satellite of Earth including the Moon the point of least distance is the perigee, for objects in Lunar orbit, the point of least distance is the pericynthion and the greatest distance the apocynthion. For any orbits around a center of mass, there are the terms pericenter and apocenter, periapsis and apoapsis are equivalent alternatives. A straight line connecting the pericenter and apocenter is the line of apsides and this is the major axis of the ellipse, its greatest diameter. For a two-body system the center of mass of the lies on this line at one of the two foci of the ellipse. When one body is larger than the other it may be taken to be at this focus. Historically, in systems, apsides were measured from the center of the Earth. In orbital mechanics, the apsis technically refers to the distance measured between the centers of mass of the central and orbiting body. However, in the case of spacecraft, the family of terms are used to refer to the orbital altitude of the spacecraft from the surface of the central body. The arithmetic mean of the two limiting distances is the length of the axis a. The geometric mean of the two distances is the length of the semi-minor axis b, the geometric mean of the two limiting speeds is −2 ε = μ a which is the speed of a body in a circular orbit whose radius is a. The words pericenter and apocenter are often seen, although periapsis/apoapsis are preferred in technical usage, various related terms are used for other celestial objects. The -gee, -helion and -astron and -galacticon forms are used in the astronomical literature when referring to the Earth, Sun, stars. The suffix -jove is occasionally used for Jupiter, while -saturnium has very rarely used in the last 50 years for Saturn. The -gee form is used as a generic closest approach to planet term instead of specifically applying to the Earth. During the Apollo program, the terms pericynthion and apocynthion were used when referring to the Moon, regarding black holes, the term peri/apomelasma was used by physicist Geoffrey A. Landis in 1998 before peri/aponigricon appeared in the scientific literature in 2002

6.
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

7.
Orbital eccentricity
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The orbital eccentricity of an astronomical object is a parameter that determines the amount by which its orbit around another body deviates from a perfect circle. A value of 0 is an orbit, values between 0 and 1 form an elliptical orbit,1 is a parabolic escape orbit. The term derives its name from the parameters of conic sections and it is normally used for the isolated two-body problem, but extensions exist for objects following a rosette orbit through the galaxy. In a two-body problem with inverse-square-law force, every orbit is a Kepler orbit, the eccentricity of this Kepler orbit is a non-negative number that defines its shape. The limit case between an ellipse and a hyperbola, when e equals 1, is parabola, radial trajectories are classified as elliptic, parabolic, or hyperbolic based on the energy of the orbit, not the eccentricity. Radial orbits have zero angular momentum and hence eccentricity equal to one, keeping the energy constant and reducing the angular momentum, elliptic, parabolic, and hyperbolic orbits each tend to the corresponding type of radial trajectory while e tends to 1. For a repulsive force only the trajectory, including the radial version, is applicable. For elliptical orbits, a simple proof shows that arcsin yields the projection angle of a circle to an ellipse of eccentricity e. For example, to view the eccentricity of the planet Mercury, next, tilt any circular object by that angle and the apparent ellipse projected to your eye will be of that same eccentricity. From Medieval Latin eccentricus, derived from Greek ἔκκεντρος ekkentros out of the center, from ἐκ- ek-, eccentric first appeared in English in 1551, with the definition a circle in which the earth, sun. Five years later, in 1556, a form of the word was added. The eccentricity of an orbit can be calculated from the state vectors as the magnitude of the eccentricity vector, e = | e | where. For elliptical orbits it can also be calculated from the periapsis and apoapsis since rp = a and ra = a, where a is the semimajor axis. E = r a − r p r a + r p =1 −2 r a r p +1 where, rp is the radius at periapsis. For Earths annual orbit path, ra/rp ratio = longest_radius / shortest_radius ≈1.034 relative to center point of path, the eccentricity of the Earths orbit is currently about 0.0167, the Earths orbit is nearly circular. Venus and Neptune have even lower eccentricity, over hundreds of thousands of years, the eccentricity of the Earths orbit varies from nearly 0.0034 to almost 0.058 as a result of gravitational attractions among the planets. The table lists the values for all planets and dwarf planets, Mercury has the greatest orbital eccentricity of any planet in the Solar System. Such eccentricity is sufficient for Mercury to receive twice as much solar irradiation at perihelion compared to aphelion, before its demotion from planet status in 2006, Pluto was considered to be the planet with the most eccentric orbit