Astrophysics (index)

Orbital Element

(parameters describing an orbit)

Orbital Elements are parameters that describe the orbit according to a reference plane and direction through that plane.

The shape and size:

  • Eccentricity - a ratio describing the shape of the Ellipse.
  • Semimajor Axis - half the longest diameter of the ellipse.

References for the orientation:

  • Plane of Reference - a plane used as a basis for describing the orbit, e.g., the Ecliptic.
  • Reference Direction - a direction through the Plane of Reference for describing the orbit.

The elements describing the orbit's orientation use the line formed by the intersection of the orbit's plane with the reference plane (the "Intersection" below):

  • Ascending Node - the point on the Intersection through which the orbiting body passes from South to North.
  • Longitude of Ascending Node - angle from the reference direction to the Ascending Node on the intersection.
  • Inclination - angle at the intersection between the Reference Direction and the orbit's plane.
  • Argument of Periapsis - angle through the Reference Plane from the Reference Direction to the intersection/Ascending Node.
  • True Anomaly - angle around the orbit's plane from the intersection/Ascending Node to a line from the origin to the body's current location.

(orbits,celestial mechanics)