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Theargument of periapsis (also calledargument of perifocus orargument of pericenter), symbolized asω (omega), is one of theorbital elements of anorbiting body. Parametrically,ω is the angle from the body'sascending node to itsperiapsis, measured in the direction of motion.
For specific types of orbits, terms such asargument of perihelion (forheliocentric orbits),argument of perigee (forgeocentric orbits),argument of periastron (for orbits around stars), and so on, may be used (seeapsis for more information).
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. An argument of periapsis of 90° means that the orbiting body will reach periapsis at its northmost distance from the plane of reference.
Adding the argument of periapsis to thelongitude of the ascending node gives thelongitude of the periapsis. 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".
Inastrodynamics theargument of periapsisω can be calculated as follows:
where:
In the case ofequatorial orbits (which have no ascending node), the argument is strictly undefined. However, if the convention of setting the longitude of the ascending node Ω to 0 is followed, then the value ofω follows from the two-dimensional case:
where:
In the case of circular orbits it is often assumed that the periapsis is placed at the ascending node and thereforeω = 0. However, in the professional exoplanet community,ω = 90° is more often assumed for circular orbits, which has the advantage that the time of a planet's inferior conjunction (which would be the time the planet would transit if the geometry were favorable) is equal to the time of its periastron.[1][2][3]
