An Etymological Dictionary of Astronomy and Astrophysics
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The angular distance between the → ascending node of an object orbiting a → primary and its periapsis measured from the primary. Argument of periapsis is measured in the → orbital plane in the direction of motion. It is one of the → orbital elements. See also → argument of perigee, → argument of perihelion.

→ argument; → perigee.

The point in an orbit of a body where it is closest to its primary; opposite of → apoapsis.

→ peri- + → apsis.

One of the → orbital elements, the time when the → secondary body reaches → periapsis.

→ time; → periapsis; → passage.