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Noun 1. orbit - the (usually elliptical) path described by one celestial body in its revolution about another; "he plotted the orbit of the moon" celestial orbit apoapsis, point of apoapsis - (astronomy) the point in an orbit farthest from the body being orbited geosynchronous orbit - a circular orbit around the Earth having a period of 24 hours itinerary, route, path - an established line of travel or access periapsis, point of periapsis - (astronomy) the point in an orbit closest to the body being orbited 2. orbit - a particular environment or walk of life; "his social sphere is limited"; "it was a closed area of employment"; "he's out of my orbit" arena, domain, sphere, area, field environment - the totality of surrounding conditions; "he longed for the comfortable environment of his living room" distaff - the sphere of work by women front - a sphere of activity involving effort; "the Japanese were active last week on the diplomatic front"; "they advertise on many different fronts" kingdom, realm, land - a domain in which something is dominant; "the untroubled kingdom of reason"; "a land of make-believe"; "the rise of the realm of cotton in the south" lap - an area of control or responsibility; "the job fell right in my lap" political arena, political sphere - a sphere of intense political activity preserve - a domain that seems to be specially reserved for someone; "medicine is no longer a male preserve" province, responsibility - the proper sphere or extent of your activities; "it was his province to take care of himself" 3. orbit - an area in which something acts or operates or has power or control: "the range of a supersonic jet"; "a piano has a greater range than the human voice"; "the ambit of municipal legislation"; "within the compass of this article"; "within the scope of an investigation"; "outside the reach of the law"; "in the political orbit of a world power" ambit, range, scope, reach, compass

extent - the distance or area or volume over which something extends; "the vast extent of the desert"; "an orchard of considerable extent" approximate range, ballpark - near to the scope or range of something; "his answer wasn't even in the right ballpark" confines - a bounded scope; "he stayed within the confines of the city" contrast - the range of optical density and tone on a photographic negative or print (or the extent to which adjacent areas on a television screen differ in brightness) internationality, internationalism - quality of being international in scope; "he applauded the internationality of scientific terminology" latitude - scope for freedom of e.g. action or thought; freedom from restriction purview, horizon, view - the range of interest or activity that can be anticipated; "It is beyond the horizon of present knowledge" expanse, sweep - a wide scope; "the sweep of the plains" gamut - a complete extent or range: "a face that expressed a gamut of emotions" spectrum - a broad range of related objects or values or qualities or ideas or activities palette, pallet - the range of colour characteristic of a particular artist or painting or school of art 4. orbit - the path of an electron around the nucleus of an atom electron orbit itinerary, route, path - an established line of travel or access

1. often Moon The natural satellite of Earth, visible by reflection of sunlight and having a slightly elliptical orbit, approximately 356,000 kilometers (221,600 miles) distant at perigee and 406,997 kilometers (252,950 miles) at apogee. Its mean diameter is 3,475 kilometers (2,160 miles), its mass approximately one eightieth that of Earth, and its average period of revolution around Earth 29 days 12 hours 44 minutes calculated with respect to the sun. 2. A natural satellite revolving around a planet. 3. The moon as it appears at a particular time in its cycle of phases: a gibbous moon. 4. A month, especially a lunar month. 5. A disk, globe, or crescent resembling the natural satellite of Earth.

moon, natural satellite of a planet (see satellite, natural) or dwarf planet, in

particular, the single natural satellite of the earth.

The Earth-Moon System
The moon is the earth's nearest neighbor in space. In addition to its proximity, the moon is also exceptional in that it is quite massive compared to the earth itself, the ratio of their masses being far larger than the similar ratios of other natural satellites to the planets they orbit (though that of Charon and the dwarf planet Pluto exceeds that of the moon and earth). For this reason, the earth-moon system is sometimes considered a double planet. It is the center of the earth-moon system, rather than the center of the earth itself, that describes an elliptical orbit around the sun in accordance with Kepler's laws. It is also more accurate to say that the earth and moon together revolve about their common center of mass, rather than saying that the moon revolves about the earth. This common center of mass lies beneath the earth's surface, about 3,000 mi (4800 km) from the earth's center. The Lunar Month The moon was studied, and its apparent motions through the sky recorded, beginning in ancient times. The Babylonians and the Maya, for example, had remarkably precise calendars for eclipses and other astronomical

events. Astronomers now recognize different kinds of months, such as the synodic month of 29 days, 12 hr, 44 min, the period of the lunar phases, and the sidereal month of 27 days, 7 hr, 43 min, the period of lunar revolution around the earth. The Lunar Orbit As seen from above the earth's north pole, the moon moves in a counterclockwise direction with an average orbital speed of about 0.6 mi/sec (1 km/sec). Because the lunar orbit is elliptical, the distance between the earth and the moon varies periodically as the moon revolves in its orbit. At perigee, when the moon is nearest the earth, the distance is about 227,000 mi (365,000 km); at apogee, when the moon is farthest from the earth, the distance is about 254,000 mi (409,000 km). The average distance is about 240,000 mi (385,000 km), or about 60 times the radius of the earth itself. The plane of the moon's orbit is tilted, or inclined, at an angle of about 5° with respect to the ecliptic. The line dividing the bright and dark portions of the moon is called the terminator. Retarded Lunar Motion Due to the earth's rotation, the moon appears to rise in the east and set in the west, like all other heavenly bodies; however, the moon's own orbital motion carries it eastward against the stars. This apparent motion is much more rapid than the similar motion of the sun. Hence the moon appears to overtake the sun and rises on an average of 50 minutes later each night. There are many variations in this retardation according to latitude and time of year. In much of the Northern Hemisphere, at the autumnal equinox, the harvest moon occurs; moonrise and sunset nearly coincide for several days around full moon. The next succeeding full moon, called the hunter's moon, also shows this coincidence. Solar and Lunar Eclipses Although an optical illusion causes the moon to appear larger when it is near the horizon than when it is near the zenith, the true angular size of the moon's diameter is about 1-2°, which also happens to be the sun's apparent diameter. This coincidence makes possible total eclipses of the sun in which the solar disk is exactly covered by the disk of the moon. An eclipse of the moon occurs when the earth's shadow falls onto the moon, temporarily blocking the sunlight that causes the moon to shine. Eclipses

can occur only when the moon, sun, and earth are arranged along a straight line—lunar eclipses at full moon and solar eclipses at new moon. Tidal Influence of the Moon The gravitational influence of the moon is chiefly responsible for the tides of the earth's oceans, the twice-daily rise and fall of sea level. The ocean tides are caused by the flow of water toward the two points on the earth's surface that are instantaneously directly beneath the moon and directly opposite the moon. Because of frictional drag, the earth's rotation carries the two tidal bulges slightly forward of the line connecting earth and moon. The resulting torque slows the earth's rotation while increasing the moon's orbital velocity. As a result, the day is getting longer and the moon is moving farther away from the earth. The moon also raises much smaller tides in the solid crust of the earth, deforming its shape. The tidal influence of the earth on the moon was responsible for making the moon's periods of rotation and revolution equal, so that the same side of the moon always faces earth.

Physical Characteristics
The study of the moon's surface increased with the invention of the telescope by Galileo in 1610 and culminated in 1969 when the first human actually set foot on the moon's surface. The physical characteristics and surface of the moon thus have been studied telescopically, photographically, and more recently by instruments carried by manned and unmanned spacecraft (see space exploration). The moon's diameter is about 2,160 mi (3,476 km) at the moon's equator, somewhat more than 1-4 the earth's diameter. The moon has about 1-81 the mass of the earth and is 3-5 as dense. On the moon's surface the force of gravitation is about 1-6 that on earth. It has been established that the moon completely lacks an atmosphere and, despite some tantalizing hints that there might be ice under the surface dust in shaded portions of Shackleton Crater (near the moon's south pole), there is no definite evidence of water. The surface temperature rises above 100°C; (212°F;) at lunar noon and sinks below −155°C; (−247°F;) at night. The gross surface features of the moon are visible to the unaided eye and were first studied telescopically in 1610 by Galileo. Surface Features

The lunar surface is divided into the mountainous highlands and the large, roughly circular plains called maria (sing. mare; from Lat.,=sea) by early astronomers, who erroneously believed them to be bodies of water. The smooth floors of the maria, varying from flat to gently undulating, are covered by a thin layer of powdered rock that darkens them and accounts for the moon's low albedo (only 7% of the incident sunlight is reflected back, the rest being absorbed). The brighter regions on the moon are the mountainous highlands, where the terrain is rough and strewn with rocky rubble. The lunar mountain ranges, with heights up to 25,000 ft (7800 m), are comparable to the highest mountains on earth but in general are not very steep. The highlands are densely scarred by thousands of craters— shallow circular depressions, usually ringed by well-defined walls and often possessing a central peak. Craters range in diameter from a few feet to many miles, and in some regions there are so many that they overlap or several smaller craters lie within a large crater. Craters are also found on the maria, although there are nowhere near as many as in the lunar highlands. Other prominent surface features include the rilles and rays. Rilles are sinuous, canyonlike clefts found near the edges of mountain ranges. Rays are bright streaks radiating outward from certain craters, such as Tycho. Mare and highland rocks differ in both appearance and chemical content. For example, mare rocks are richer in iron and poorer in aluminum than highland rocks. The maria consist largely of basalt, i.e., igneous rock formed from magma. In the highlands the majority of the rocks are breccias—conglomerates formed from basaltic rock and often studded with small, green, glassy spheres. These spheres probably were formed as the spray of molten rock, originally melted by the heat of meteorite impact, recongealed in midflight. The exposure ages of some rocks (the time their surfaces have been exposed to the action of cosmic rays that produce radioactive isotopes) are as short as 50 million years, much shorter than their crystallization ages. These rocks may have been shifted in position by meteorite impact or seismic activity (moonquakes). However, present lunar seismic activity is very low, corroborating the image of the moon as an essentially static, nonevolving world. Internal Structure Diffraction of seismic waves provided the first clear-cut evidence for a lunar crust, mantle, and core analogous to those of the earth. The lunar crust is

about 45 mi (70 km) thick, making the moon a rigid solid to a greater depth than the earth. The inner core has a radius of about 600 mi (1,000 km), about 2-3 of the radius of the moon itself. The internal temperature decreases from 830°C; (1,530°F;) at the center to 170°C; (340°F;) near the surface. The heat traveling outward near the lunar surface is about half that of the earth but still twice that predicted by current theory. This heat flow is directly related to the rate of internal energy production, so that the internal temperature profile provides information about long-lived radio isotopes and the moon's thermal evolution. The heat-flow measurements indicate that the moon's radioactive content is higher than that of the earth. The moon's magnetic field is a million times weaker than that of the earth, but it varies by a factor of 20 from point to point on the surface. Certain rocks retain a high magnetization, indicating that they crystallized in the presence of magnetic fields much higher than those presently existing on the moon. Mascons are large concentrations of unusually high density that are located below certain of the circular maria. The mascons may have been created by the implantation of very dense, iron-rich meteorites, whose impact formed the mare basins themselves. Formation and Evolution The moon probably formed by the cold accretion of small particles about 4.6 billion years ago at the same time that the rest of the solar system formed; thus, it is now believed that the moon was never in an entirely molten state. The crust, showing pronounced chemical differentiation, formed early. Subsequent impact of very large meteorites depressed the mare basins, at the same time thrusting up the surrounding crust to form the highlands. The mare basins later filled with lava flow, which in turn was covered by a thin layer of lunar "soil"—fine rock dust pulverized by the very slow mechanisms of lunar erosion (thermal cycling, solar wind, and micrometeorites). The craters were probably also formed by meteorite bombardment rather than by internal volcanic action as once believed. The rays surrounding the craters are material ejected during the impacts that formed the craters. The moon's rock types are correlated with its major geological periods.

Star around which the components of the solar system revolve. It is about five billion years old and is the dominant body of the system, with more than 99% of its mass. It converts five million tons of matter into energy every second by nuclear fusion reactions in its core, producing neutrinos (see solar neutrino problem) and solar radiation. The small amount of this energy that penetrates Earth’s atmosphere provides the light and heat that support life. A sphere of luminous gas 864,950 mi (1,392,000 km) in diameter, the Sun has about 330,000 times the mass of Earth. Its core temperature is close to 27 million °F (15 million °C) and its surface temperature about 10,000 °F (6,000 °C). The Sun, a spectral type G (yellow) star, has fairly average properties for a main-sequence star (see Hertzsprung-Russell diagram). It rotates at different rates at different latitudes; one rotation takes 36 days at the poles but only 25 days at the equator. The visible surface, or photosphere, is in constant motion, with the number and position of sunspots changing in a regular solar cycle. External phenomena include magnetic activity extending into the chromosphere and corona, solar flares, solar prominences, and the solar wind. Effects on Earth include auroras and disruption of radio communications and power-transmission lines. Despite its activity, the Sun appears to have remained relatively unchanged for billions of years. See also eclipse; heliopause
sun [sən]

(astronomy) The star about which the earth revolves; it is a globe of gas 8.65 × 105miles (1.392 × 106kilometers) in diameter, held together by its own gravity; thermonuclear reactions take place in the deep interior of the sun converting hydrogen into helium releasing energy which streams out. Also known as Sol.

(pl n t) a. In the traditional model of solar systems, a celestial body larger than an asteroid or comet, illuminated by light from a star, such as the sun, around which it revolves. b. A celestial body that orbits the sun, has sufficient mass to assume nearly a round shape, clears out dust and debris from the neighborhood around its orbit, and is not a satellite of another planet. 2. One of the seven celestial bodies, Mercury, Venus, the moon, the sun, Mars, Jupiter, and Saturn, visible to the naked eye and thought by ancient astronomers to revolve in the heavens about a fixed Earth and among fixed stars. 3. One of the seven revolving astrological celestial bodies that in conjunction with the stars are believed to influence human affairs and personalities.

Any large natural body that orbits the Sun or another star (see planets of other stars) and that is not radiating energy from internal nuclear fusion; dwarf planets, comets, asteroids, meteoroids (see meteor), and naturalsatellites are excluded. The word planet comes from the Greek for “wanderer,” because the planets' positions change relative to those of the stars. The eight (formerly nine) recognized planets that orbit the Sun are, in order of increasing distance, Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. The first four are called terrestrial planets and the next four giant, or Jovian, planets. The terrestrial planets (of which Earth is the largest) are rocky with comparatively thin or negligible atmospheres. The Sun's heat is thought to have prevented the abundant volatile substances in the solar nebula from condensing in them as they formed. The giant planets formed farther out, where the gases were cool enough to condense, so the planets grew very massive and accumulated huge atmospheres of light gases, mainly hydrogen and helium. None of the giant planets has an accessible surface; their gaseous atmospheres gradually merge with their liquid interiors. Pluto, although it was recognized as a planet for decades, is distinct from either group, being much smaller than any of the eight planets and resembling a giant comet nucleus. In 2006 the International Astronomical Union demoted Pluto to the category of dwarf planet, which reflected astronomers' conclusion that it is a very large member of the Kuiper belt. Additional bodies rivaling or exceeding Pluto in

size (e.g., Eris) exist in the outer region of the solar system. The term minor planet is sometimes used to refer to any of the asteroids that lie mostly between the terrestrial and the giant planets. In astrology great importance is placed on the planets' positions in the 12 constellations of thezodiac. See also planetesimal; solar system.

1. any of the nine celestial bodies, Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, or Pluto, that revolve around the sun in elliptical orbits and are illuminated by light from the sun 2. any other celestial body revolving around a star, illuminated by light from that star 3. Astrology any of the planets of the solar system, excluding the earth but including the sun and moon, each thought to rule one or sometimes two signs of the zodiac

earth, in chemistry
in chemistry, metallic oxide not readily reducible by chemical means, e.g., alkaline earths, rare earths, and alumina. The name is also applied to certain absorbent clays, e.g., fuller's earth, and to other compounds, e.g., carbonates, silicates, or hydroxides. Many earths were once thought to be elements. A. L. Lavoisier was first to suspect that they might be compounds of more basic elements. Earth was one of the four "roots" of the Greek philosopher Empedocles, the other three being air, water, and fire. These substances were first called elements (stoicheia) by Plato.
Earth's interior may be identified in two distinct ways. In chemical terms, it has three basic … (credit: © Merriam-Webster Inc.)

Third planet in distance outward from the Sun. Believed to be about 4.6 billion years old, it is some 92,960,000 mi (149,600,000 km) from the Sun. It orbits the Sun at a speed of 18.5 mi (29.8 km) per second, making one complete revolution in 365.25 days. As it revolves, it rotates on its axis once every 23 hours 56 minutes 4 seconds. The fifth largest planet of the solar system, it has an equatorial circumference of 24,902 mi (40,076 km). Its total surface area is roughly 197,000,000 sq mi (509,600,000 sq km), of which about 29% is land. Earth's atmosphere consists of a mixture

of gases, chiefly nitrogen and oxygen. Its only natural satellite, the Moon, orbits the planet at a distance of about 238,860 mi (384,400 km). Earth's surface is traditionally subdivided into seven continental masses:Africa, Antarctica, Asia, Australia, Europe, North America, and South America. These continents are surrounded by four major bodies of water: the Arctic, Atlantic, Indian, andPacific oceans. Broadly speaking, Earth's interior consists of two regions: a corecomposed largely of molten, iron-rich metallic alloy; and a rocky shell of silicate minerals comprising both the mantle and crust (see also Moho; lithosphere). Fluid motions in the electrically conductive outer core generate a magnetic field around Earth that is responsible for the Van Allen radiation belts. According to the theory of plate tectonics, the crust and upper mantle are divided into a number of large and small plates that float on and travel independently of the lower mantle. Plate motions are responsible forcontinental drift and seafloor spreading and for most volcanic and seismic activity on Earth.

The Origin of the Earth
The earth is estimated to be 4.5 billion to 5 billion years old, based on radioactive dating of lunar rocks and meteorites, which are thought to have formed at the same time. The origin of the earth continues to be controversial. Among the theories as to its origin, the most prominent are gravitational condensation hypotheses, which suggest that the entire solar system was formed at one time in a single series of processes resulting in the accumulation of diffuse interstellar gases and dust into a solar system of discrete bodies. Older and now generally discredited theories invoked extraordinary events, such as the gravitational disruption of a star passing close to the sun or the explosion of a companion star to the sun.

star, hot incandescent sphere of gas, held together by its own gravitation, and emitting light and other forms ofelectromagnetic radiation whose ultimate source is nuclear energy.

Any massive celestial body of gas that shines by radiant energy generated inside it. The Milky Way Galaxy contains hundreds of billions of stars; only a very small fraction are visible to the unaided eye. The closest star to Earth is the Sun. The closest star to the Sun is about 4.2 light-years away; the most distant are in galaxies billions of light-years away. Single stars such as the Sun are the minority; most stars occur in pairs and multiple systems (see binary star). Stars also associate by their mutual gravity in larger assemblages called clusters (see globular cluster; open cluster).Constellations consist not of such groupings but of stars in the same direction as seen from Earth. Stars vary greatly in brightness (magnitude), colour, temperature, mass, size, chemical composition, and age. In nearly all, hydrogen is the most abundant element. Stars are classified by their spectra (see spectrum), from blue-white to red, as O, B, A, F, G, K, or M; the Sun is a spectral type G star. Generalizations on the nature and evolution of stars can be made from correlations between certain properties and from statistical results (see Hertzsprung-Russell diagram). A star forms when a portion of a dense interstellar cloud of hydrogen and dust grains collapses from its own gravity. As the cloud condenses, its density and internal temperature increase until it is hot enough to trigger nuclear fusion in its core (if not, it becomes abrown dwarf). After hydrogen is exhausted in the core from nuclear burning, the core shrinks and heats up while the star's outer layers expand significantly and cool, and the star becomes a red giant. The final stages of a star's evolution, when it no longer produces enough energy to counteract its own gravity, depend largely on its mass and whether it is a component of a close binary system (see black hole; neutron star; nova; pulsar; supernova; white dwarf star). Some stars other than the Sun are known to have one or more planets (see extrasolar planet). See also Cepheid variable; dwarf star;eclipsing variable star; flare star; giant star; Populations I and II; supergiant star; T Tauri star; variable star.

Properties of Stars
Stars differ widely in mass, size, temperature, and total energy output, or luminosity. The sun, a typical star, has a mass of about 2 × 1033 grams, a radius of about 7 × 1010 cm, a surface temperature of about 6,000°C;, and a luminosity of about 4 × 1033 erg/sec. More than 90% of all stars have masses between one tenth and 50 times that of the sun. Other stellar quantities vary over a much larger range. The most luminous stars (excluding supernovas) are about ten million times more powerful than the sun, while the least luminous are only one hundredth as powerful. Red giants, the largest stars, are fifteen-hundred times greater in size than the sun; if one were placed at the sun's position, it would stretch to halfway between Jupiter and Saturn. At the opposite extreme, white dwarfs are no larger than the earth, and neutron stars are only a few kilometers in radius. The visible stars are divided into six classes according to apparent brightness; the brightest are first magnitude and the faintest are sixth magnitude. The stars differ in apparent brightness both because they lie at different distances from us and because they vary in actual or intrinsic brightness. Variable stars do not shine steadily but fluctuate in either a regular or irregular fashion. The supernova, or exploding star, is the most spectacular variable star; the eclipsing binary, where the two stars alternately hide and then reinforce each other's light, is not a true variable. Light received from a star consists of a spectrum of wavelengths; the hotter the star, the shorter the wavelength at which the light is most intense. The color of a star is closely related to its surface temperature. Red stars have surface temperatures around 3,000°C; and blue-white stars have surface temperatures above 20,000°C; (see spectral class).

Stellar Structure and Stellar Evolution
The theory of stellar structure applies the laws of physics to calculation of the equilibrium configurations of stars. According to this theory, the mass and chemical composition of a star determine all its other characteristics. Because most stars are more than 90% hydrogen, variations in chemical composition are small and have a small effect. Variation in mass is the main factor; a doubling in mass increases the luminosity more than 10 times. For a star to be stable, the compressive force of gravitation must be

exactly balanced by the tendency of the gas to expand. Thus, the size and temperature of a star are important, interrelated factors. Despite the tremendous pressure generated by the massive layers above it, the central region, or core, of a star remains gaseous. This is possible because the core has a temperature of millions of degrees. At this temperature, nuclear energy is released by the fusion of hydrogen to form helium; the principle is the same as that of the hydrogen bomb. By the time nuclear energy reaches the surface of the star, it has been largely converted into visible light with a spectrum characteristic of a very hot body (see black body). The theory of stellar evolution states that a star must change as it consumes its hydrogen in the nuclear reactions that power it. Ultimately each star must die, rarely in a supernova explosion, when its capability for nuclear reactions is exhausted. The heavy atoms created in supernovas (seenucleosynthesis) are spewed out to become part of the interstellar matter from which new stars are continuously formed.

Location and Motion of Stars
The universe contains billions of galaxies, and each galaxy contains billions of stars. The stars visible to the unaided eye are all in our own galaxy, the Milky Way. Stars are not spread uniformly through a galaxy. They are frequently bunched together in star clusters of as many as 100,000 stars. Many stars that appear as single points of light in even the most powerful telescopes are actually systems of two or more stars orbiting one another, bound together by their mutual gravitational attraction; the binary stars are most common among these multiple star systems. In ancient times, the stars were believed to be motionless; their fixed patterns in the sky were designated as theconstellations. It is now known that the stars move through space, although their motion is too small to be detected during a human lifetime without exacting measurements. From the observed proper motion (change in apparent position on thecelestial sphere), distance of the star from the earth, and radial velocity (motion along the line of sight), the true velocity of a star through space can be determined. See also brown dwarf.

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