### orbital element

(parameters describing an orbit)

Orbital elements are parameters that describe an orbit (the path of an astronomical object gravitationally bound to another, particularly, a repeating pattern) according to a reference plane and a 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 the side declared "down" to that declared "up". (in the case of solar system orbits, the side to which Earth north points is also known as the north node).
• longitude of ascending node - angle from the reference direction to the ascending node on the intersection.
• descending node - similar point for the orbiting passing to the opposite "downward" (or south node in the solar system).
• line of nodes - line through the reference plane through which the orbit passes.
• 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)
http://en.wikipedia.org/wiki/Orbital_elements

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