NASA - National Aeronautics and Space Administration

An orbit is the path an object takes as it goes around another object. The word orbit is used in astronomy, the study of stars, planets, and other objects in space. In astronomy, an object in orbit is controlled by the gravity, or pulling force, of another object. The Earth and the other planets travel in orbit around the sun. The moon moves in orbit around the Earth. A planet's orbit makes an oval-shaped curve called an ellipse.

Image to left: Satellites used for research and communications travel in a path, or orbit, around the Earth. Credit: World Book illustration by Roberta Polfus

Scientists who study stars and planets call the bigger object or planet the primary and the smaller object or planet the secondary. Spacecraft can become secondaries of the Earth or another planet. Spacecraft that leave their orbit around the Earth and head toward another planet travel in a slightly curved orbit.

How to cite this article: To cite this article, World Book recommends the following format: "Orbit." The World Book Student Discovery Encyclopedia. Chicago: World Book, Inc., 2005.

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