Plane of reference
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A term used in celestial mechanics, the plane of reference is the plane by means of which orbital elements (positions) are defined. The two main orbital elements that are measured with respect to the plane of reference are the inclination and the longitude of the ascending node.
Depending on the type of body being described, there are four different kinds of reference planes that are typically used:
- Ecliptic or invariable plane for planets, asteroids, comets, etc. within the solar system, as these bodies generally have orbits that lie close to the ecliptic.
- Equator of orbited body for 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 on the celestial sphere where the plane crosses the prime hour circle (the hour circle occupied by the First Point of Aries).
