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in astronomy, path in space described by a body revolving about a second body where the motion of the orbiting bodies is dominated by their mutual gravitational attraction. Within the solar system, planets, dwarf planets, asteroids, and comets orbit the sun and satellites orbit the planets and other bodies.

Planetary Orbits

From earliest times, astronomers assumed that the orbits in which the planets moved were circular; yet the numerous catalogs of measurements compiled especially during the 16th cent. did not fit this theory. At the beginning of the 17th cent., Johannes Kepler stated three laws of planetary motion that explained the observed data: the orbit of each planet is an ellipse with the sun at one focus; the speed of a planet varies in such a way that an imaginary line drawn from the planet to the sun sweeps out equal areas in equal amounts of time; and the ratio of the squares of the periods of revolution of any two planets is equal to the ratio of the cubes of their average distances from the sun. The orbits of the solar planets, while elliptical, are almost circular; on the other hand, the orbits of many of the extrasolar planets discovered during the 1990s are highly elliptical.

After the laws of planetary motion were established, astronomers developed the means of determining the size, shape, and relative position in space of a planet's orbit. The size and shape of an orbit are specified by its semimajor axis and by its eccentricity. The semimajor axis is a length equal to half the greatest diameter of the orbit. The eccentricity is the distance of the sun from the center of the orbit divided by the length of the orbit's semimajor axis; this value is a measure of how elliptical the orbit is. The position of the orbit in space, relative to the earth, is determined by three factors: (1) the inclinationinclination,
in astronomy, the angle of intersection between two planes, one of which is an orbital plane. The inclination of the plane of the moon's orbit is 5°9' with respect to the plane of the ecliptic (the plane of the earth's orbit around the sun).
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, or tilt, of the plane of the planet's orbit to the plane of the earth's orbit (the ecliptic); (2) the longitude of the planet's ascending node (the point where the planet cuts the ecliptic moving from south to north); and (3) the longitude of the planet's perihelion point (point at which it is nearest the sun; see apsisapsis
(pl. apsides), point in the orbit of a body where the body is neither approaching nor receding from another body about which it revolves. Any elliptical orbit has two apsides.
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These quantities, which determine the size, shape, and position of a planet's orbit, are known as the orbital elements. If only the sun influenced the planet in its orbit, then by knowing the orbital elements plus its position at some particular time, one could calculate its position at any later time. However, the gravitational attractions of bodies other than the sun cause perturbationsperturbation
, in astronomy and physics, small force or other influence that modifies the otherwise simple motion of some object. The term is also used for the effect produced by the perturbation, e.g., a change in the object's energy or path of motion.
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 in the planet's motions that can make the orbit shift, or precess, in space or can cause the planet to wobble slightly. Once these perturbations have been calculated one can closely determine its position for any future date over long periods of time. Modern methods for computing the orbit of a planet or other body have been refined from methods developed by NewtonNewton, Sir Isaac,
1642–1727, English mathematician and natural philosopher (physicist), who is considered by many the greatest scientist that ever lived. Early Life and Work
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, LaplaceLaplace, Pierre Simon, marquis de
, 1749–1827, French astronomer and mathematician. At 18 he went to Paris, proved his gift for mathematical analysis to Jean le Rond d'Alembert, and was made professor of mathematics in the École militaire of Paris.
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, and GaussGauss, Carl Friedrich
, born Johann Friederich Carl Gauss, 1777–1855, German mathematician, physicist, and astronomer. Gauss was educated at the Caroline College, Brunswick, and the Univ.
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, in which all the needed quantities are acquired from three separate observations of the planet's apparent position.

Nonplanetary Orbits

The laws of planetary orbits also apply to the orbits of comets, natural satellites, artificial satellites, and space probes. The orbits of comets are very elongated; some are long ellipses, some are nearly parabolic (see parabolaparabola
, plane curve consisting of all points equidistant from a given fixed point (focus) and a given fixed line (directrix). It is the conic section cut by a plane parallel to one of the elements of the cone.
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), and some may be hyperbolic. When the orbit of a newly discovered comet is calculated, it is first assumed to be a parabola and then corrected to its actual shape when more measured positions are obtained. Natural satellites that are close to their primaries tend to have nearly circular orbits in the same plane as that of the planet's equator, while more distant satellites may have quite eccentric orbits with large inclinations to the planet's equatorial plane. Because of the moon's proximity to the earth and its large relative mass, 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 lawsKepler's laws,
three mathematical statements formulated by the German astronomer Johannes Kepler that accurately describe the revolutions of the planets around the sun. Kepler's laws opened the way for the development of celestial mechanics, i.e.
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. All of the planets and most of the satellites in the solar system move in the same direction in their orbits, counterclockwise as viewed from the north celestial pole; some satellites, probably captured asteroids, have retrograde motionretrograde motion,
in astronomy, real or apparent movement of a planet, dwarf planet, moon, asteroid, or comet from east to west relative to the fixed stars. The most common direction of motion in the solar system, both for orbital revolution and axial rotation, is from west to
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, i.e., they revolve in a clockwise direction.


(or -bit) The path followed by a celestial object or an artificial satellite or spaceprobe that is moving in a gravitational field. For a single object moving freely in the gravitational field of a massive body the orbit is a conic section, in actuality either elliptical or hyperbolic. Closed (repeated) orbits are elliptical, most planetary orbits being almost circular. A hyperbolic orbit results in the object escaping from the vicinity of a massive body. See also Kepler's laws; orbital elements.


(religion, spiritualism, and occult)

An orbit is the path in space that one heavenly body makes in its movement around another heavenly body. The Moon, for example, makes an orbit around Earth, while Earth and the other planets make orbits around the Sun. The technical name for the orbiting body is satellite. The orbited body is called a primary. Because primaries are also in motion, the orbits described by satellites are elliptical rather than circular.

Satellites form stable orbits by counterbalancing two forces—their movement away from the primary and the force of gravity drawing them back toward the primary. In other words, in the absence of gravity a satellite would move in a straight line, which would soon take it away from its primary; in the absence of satellite motion, gravity would draw a satellite and its primary together until they collided.


Robinson, J. Hedley, and James Muirden. Astronomy Data Book. 2d ed. New York: John Wiley & Sons, 1979.
Smoluchowski, Roman. The Solar System: The Sun, Planets, and Life. New York: Scientific American Books, 1983.


The bony cavity in the lateral front of the skull beneath the frontal bone which contains the eyeball. Also known as eye socket.
Let G be a group which operates on a set S ; the orbit of an element s of S under G is the subset of S consisting of all elements gs where g is in G.
The path of a water particle affected by wave motion; it is almost circular in deep-water waves and almost elliptical in shallow-water waves.
Any closed path followed by a particle or body, such as the orbit of a celestial body under the influence of gravity, the elliptical path followed by electrons in the Bohr theory, or the paths followed by particles in a circular particle accelerator.
More generally, any path followed by a particle, such as helical paths of particles in a magnetic field, or the parabolic path of a comet.


1. Astronomy the curved path, usually elliptical, followed by a planet, satellite, comet, etc., in its motion around another celestial body under the influence of gravitation
2. Anatomy the bony cavity containing the eyeball
3. Zoology
a. the skin surrounding the eye of a bird
b. the hollow in which lies the eye or eyestalk of an insect or other arthropod
4. Physics the path of an electron in its motion around the nucleus of an atom


A Scheme compiler.

["Orbit: An Optimising Compiler for Scheme", D.A. Kranz et al, SIGPLAN Notices 21(7):281-292 (Jul 1986)].
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