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A term used in celestial mechanics
Celestial mechanics

Celestial mechanics is the branch of astronomy that deals with the motion s of celestial objects. The field applies principles of physics, historically classical mechanics, to astronomical objects such as stars and planets to produce ephemeris data....
, the plane of reference is the plane by means of which orbital elements
Orbital elements

In celestial mechanics, the elements of an orbit are the parameters needed to specify that orbit uniquely. Orbital elements are generally considered in classical mechanics two-body systems, where a Kepler orbit is used ....
 (positions) are defined. The two main orbital elements that are measured with respect to the plane of reference are the inclination
Inclination

Inclination in general is the angle between a reference plane and another plane or Axis_of_rotation of direction. The axial tilt is expressed as the angle made by the planet's axis and a line drawn through the planet's center perpendicular to the orbital plane....
 and the longitude of the ascending node
Longitude of the ascending node

The longitude of the ascending node is one of the orbital elements used to specify the orbit of an object in space. It is the angle from a reference direction, called the origin of longitude, to the direction of the ascending node, measured in a reference plane....
.

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

On the plane of reference, a zero-point must be defined from which the angles of longitude are measured.






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A term used in celestial mechanics
Celestial mechanics

Celestial mechanics is the branch of astronomy that deals with the motion s of celestial objects. The field applies principles of physics, historically classical mechanics, to astronomical objects such as stars and planets to produce ephemeris data....
, the plane of reference is the plane by means of which orbital elements
Orbital elements

In celestial mechanics, the elements of an orbit are the parameters needed to specify that orbit uniquely. Orbital elements are generally considered in classical mechanics two-body systems, where a Kepler orbit is used ....
 (positions) are defined. The two main orbital elements that are measured with respect to the plane of reference are the inclination
Inclination

Inclination in general is the angle between a reference plane and another plane or Axis_of_rotation of direction. The axial tilt is expressed as the angle made by the planet's axis and a line drawn through the planet's center perpendicular to the orbital plane....
 and the longitude of the ascending node
Longitude of the ascending node

The longitude of the ascending node is one of the orbital elements used to specify the orbit of an object in space. It is the angle from a reference direction, called the origin of longitude, to the direction of the ascending node, measured in a reference plane....
.

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

    The ecliptic is the apparent path that the Sun traces out in the sky during the year. As it appears to move in the sky in relation to the stars, the apparent path aligns with the planets throughout the course of the year....
     or invariable plane
    Invariable plane

    The invariable plane of a planetary system is the plane passing through its barycenter which is perpendicular to its angular momentum vector . In the Solar system, about 98% of this effect is contributed by the orbital angular momenta of the four jovian planets ....
     for planets, asteroids, comets, etc. within the solar system
    Solar System

    The Solar System consists of the Sun and those Astronomical object bound to it by gravity: the eight planets and five dwarf planets, their 173 known Natural satellite, and billions of Small Solar System body....
    , as these bodies generally have orbits that lie close to the ecliptic.
  • Equator
    Equator

    The equator is the intersection of the Earth's surface with the Plane perpendicular to the Earth's rotation and containing the Earth's center of mass....
     of orbited body for satellites with small semimajor axes
  • Local Laplace plane
    Laplace plane

    The Laplace plane, named after Pierre-Simon Laplace, is defined as the mean plane occupied by the orbit of a satellite during a precession cycle....
     - satellites with intermediate-to-large semimajor axes
  • Plane tangent to celestial sphere
    Celestial sphere

    In astronomy and navigation, the celestial sphere is an imagination rotation sphere of "gigantic radius", concentric spheres and coaxial with the Earth....
     - 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).

See also

  • Fundamental plane
    Fundamental plane

    The fundamental plane may refer to:* The fundamental plane which divides a spherical coordinate system.* The fundamental plane which shows an empirical relationship between mean surface brightness, velocity dispersion and effective radius of an elliptical galaxy....