Plane of reference

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In celestial mechanics, the plane of reference is the plane from which orbital elements are defined. Specifically, the inclination and the longitude of the ascending node are the two main orbital elements that are measured with respect to the plane of reference.

Depending on the type of body being described, there are four different kinds of reference planes that are typically used:

Ecliptic - planets, asteroids, comets, etc.
Equator of orbited body - satellites with small semimajor axes
Local Laplace plane - satellites with intermediate-to-large semimajor axes
Plane tangent to celestial sphere - extrasolar objects

On the plane of reference, a zero-point must be defined from which the angles of longitude are measured. This is usually defined as the point where the plane crosses the prime hour circle on the celestial sphere (the hour circle occupied by the First Point of Aries).

Plane of reference

See also