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Planes of reference
Common planes of reference include the following:
- For a geocentric orbit, Earth's equatorial plane. In this case, non-inclined orbits are called equatorial.
- For a heliocentric orbit, the ecliptic or invariable plane. In this case, non-inclined orbits are called ecliptic.
- For an orbit outside the Solar System, the plane through the primary perpendicular to a line through the observer and the primary (called the plane of the sky).
If a reference direction from one side of the plane of reference to the other is defined, the two nodes can be distinguished. For geocentric and heliocentric orbits, the ascending node (or north node) is where the orbiting object moves north through the plane of reference, and the descending node (or south node) is where it moves south through the plane. In the case of objects outside the Solar System, the ascending node is the node where the orbiting secondary passes away from the observer, and the descending node is the node where it moves towards the observer., p. 137.
The position of the node may be used as one of a set of parameters, called orbital elements, which describe the orbit. This is done by specifying the longitude of the ascending node (or, sometimes, the longitude of the node.)
Symbols and nomenclature
|Look up anabibazon in Wiktionary, the free dictionary.|
|Look up catabibazon in Wiktionary, the free dictionary.|
The symbol of the ascending node is
For the orbit of the Moon around Earth, the plane is taken to be the ecliptic, not the equatorial plane. The gravitational pull of the Sun upon the Moon causes its nodes to gradually precess westward, completing a cycle in approximately 18.6 years.
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