Argument of periapsis

Parametrically, ω is the angle from the body's ascending node to its periapsis, measured in the direction of motion.

An argument of periapsis of 0° means that the orbiting body will be at its closest approach to the central body at the same moment that it crosses the plane of reference from South to North.

However, especially in discussions of binary stars and exoplanets, the terms "longitude of periapsis" or "longitude of periastron" are often used synonymously with "argument of periapsis".

In astrodynamics the argument of periapsis ω can be calculated as follows: where: In the case of equatorial orbits (which have no ascending node), the argument is strictly undefined.

where: In the case of circular orbits it is often assumed that the periapsis is placed at the ascending node and therefore ω = 0.