Orbit

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Orbit (astronomy and physics), path or trajectory of a body through space. A force of attraction or repulsion from a second body usually causes the path to be curved. A familiar type of orbit occurs when one body revolves around a second, strongly attracting body. In the solar system the force of gravity causes the moon to orbit about the earth and the planets to orbit about the sun, whereas in an atom electrical forces cause electrons to orbit about the nucleus. In astronomy, the orbits resulting from gravitational forces, which are discussed in this article, are the subject of the scientific field of celestial mechanics.

An orbit has the shape of a conic section—a circle, ellipse, parabola, or hyperbola—with the central body at one focus of the curve. When a satellite traces out an orbit about the center of the earth, its most distant point is called the apogee and its closest point the perigee. The perigee or apogee height of the satellite above the earth's surface is often given, instead of the perigee or apogee distance from the earth's center. The ending -gee refers to orbits about the earth; perihelion and aphelion refer to orbits about the sun; the ending -astron is used for orbits about a star; and the ending -apsis is used when the central body is not specified. The so-called line of apsides is a straight line connecting the periapsis and the apoapsis.

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