periapsiswinkel

Periapsiswinkel, also called the argument of periapsis, is the angle that specifies the orientation of an orbit within its orbital plane. It is defined as the angle, measured in the orbital plane in the direction of motion, from the ascending node to the periapsis—the point of closest approach to the central body. It is one of the six classical orbital elements used to describe a Keplerian orbit around a central mass.

The other elements are the semi-major axis, a, the eccentricity, e, the inclination, i, the longitude of the ascending node, Ω, and the time of periapsis (or mean anomaly at epoch). The combination of Ω, i, and ω fixes the orientation of the orbital ellipse in three-dimensional space, while e and a fix its size and shape. The periapsis itself lies at a true anomaly of zero; the distance at periapsis is r_p = a(1 − e). The line of apsides is the line connecting periapsis and apoapsis, and its orientation within the orbital plane is determined by ω.

For circular or near-circular orbits (e ≈ 0), ω becomes undefined because all points are effectively periapsis; in such cases ω is conventionally set to zero or not used. In perturbed or multi-body systems, ω can precess over time, changing the orientation of the orbit.

In practice, ω can be computed from state vectors using orbital mechanics formulas, or derived from observational data. The term periapsiswinkel is the German name for this orbital element.