Argument of periapsis

The argument of periapsis, ω, is one of the six orbital elements for position Designation of a Kepler ellipse in space. The angle defines the orientation of the apse ( semi-major axis). It is measured from the ascending node along the orbital plane to the periapsis.

Depending on the central body, the designations are used:

• Earth: Argument of perigee
• Sun: Argument of perihelion
• General: argument of periapsis

The position of the periapsis can also be described by the length of periapsis. This is first prograde counted from the vernal equinox along the ecliptic to the ascending node, and from there also along the path to the periapsis. It is thus the sum of the length of the ascending node Ω and the argument of periapsis ω.

In the case of Earth's orbit no path nodes are defined as the Earth's orbital plane with the ecliptic plane coincide and no intersection exists between the two. If the position of the earth's orbit - perihelion are described, this can only be done by specifying the length of the perihelion. It is simplified in this case the angle between the vernal equinox and perihelion.

• Celestial mechanics