Science Dictionary
periapsis   (pěr'ē-āp'sĭs)  Pronunciation Key 
Plural periapsides (pěr'ē-āp'sĭ-dēz')
The point at which an orbiting object is closest to the body it is orbiting. This point is sometimes given a name that is specific to the body being orbited. For example, the periapsis of an object orbiting Earth is its perigee (from gaia, the Greek word for Earth), and the periapsis of an object orbiting the Sun is its perihelion (from hēlios, the Greek word for Sun). According to Kepler's laws of planetary motion, an object is at its greatest velocity at the periapsis. Compare apoapsis.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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