# Argument of the periapsis

Argument of the periapsis ω and the other orbital elements a, e, i, Ω, and T.

The argument of the periapsis , ω , is one of the 6 orbital elements used to describe the position of a Kepler ellipse in space. The angle defines the orientation of the apse ( major semi-axis ). It is measured from the ascending node along the plane of the orbit to the periapsis. An angle of 0 ° means that an orbital object is closest to the central body when it crosses the reference plane from south to north. An angle of 90 ° means that an orbital object is closest to the central body when it has reached its northernmost point.

Depending on the central body , the following terms are common:

• Earth: argument of perigee
• Sun: argument of the perihelion
• general: argument of the periapsis

The position of the periapsis can also be described by the length of the periapsis ϖ . This is initially by the spring point rechtläufig along the ecliptic also counted up to the ascending node and from there along the path to the perihelia. So it is the sum of the length of the ascending node Ω and the argument of the periapsis ω .

In the case of the earth's orbit , no orbit nodes are defined because the plane of the earth's orbit coincides with the plane of the ecliptic and there is no line of intersection between the two. If the position of the perihelion of the earth's orbit is to be described, this can only be done by specifying the length of the perihelion. In this case it simplifies to the angle between vernal equinox and perihelion.

The position of the periapsis is also indicated by the Laplace-Runge-Lenz vector . ${\ displaystyle {\ vec {A}}}$

## literature

Hannu Karttunen, Pekka Kröger, Heikki Oja, Markku Poutanen, Karl Johan Donner: Fundamental Astronomy . 5th edition. Springer, Berlin / Heidelberg / New York 2007, ISBN 978-3-540-34143-7 , 6.4 Orbital Elements, p. 116–118 (English, Finnish: Tähtitieteen perusteet . Helsinki 2003.).