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Basic Astronomy Concepts


apoapsis, periapsis, conic section, earth satellite, hyperbola

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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.


An orbit is perturbed when the forces are more complex than those between two spherical bodies. (Kepler's laws are exact only for unperturbed orbits.) The attraction between planets causes their elliptical orbits to change with time. The sun, for example, perturbs the lunar orbit by several thousand kilometers. Atmospheric drag causes the orbit of an earth satellite to shrink, and the oblate shape of the earth causes the direction of its node and perigee to change. The theory of relativity developed by German-born American physicist Albert Einstein explains an observed perturbation in the perihelion of the planet Mercury.


Williams, James G., Ph.D.

Member, Staff, Fet Propulsion Laboratory, California Institute of Technology. Contributor to "Journal of Geophysical Research", "Icarus", and other publications.

Article key phrases:

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