periapsis

[ pĕr′ē-ăpsĭs ]

Plural periapsides (pĕr′ē-ăpsĭ-dēz′)

The point at which an orbiting object is closest to the center of mass of 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.

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The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.