Orbital plane of reference

In <a href="/facts/Celestial_mechanics/FA3uR78J">celestial mechanics</a>, the orbital plane of reference (or orbital reference plane) is the <a href="/facts/Euclidean_planes_in_three-dimensional_space/UqRo1Bs0">plane</a> used to define <a href="/facts/Orbital_elements/QCG5SI6o">orbital elements</a> (positions). The two main orbital elements that are measured with respect to the plane of reference are the <a href="/facts/Inclination/TQCFWyfd">inclination</a> and the <a href="/facts/Longitude_of_the_ascending_node/926pTlve">longitude of the ascending node</a>.
Depending on the type of body being described, there are four different kinds of reference planes that are typically used:

<ul><li>The <a href="/facts/Ecliptic/cJxXDDDW">ecliptic</a> or <a href="/facts/Invariable_plane/SSoYrjeR">invariable plane</a> for planets, asteroids, comets, etc. within the <a href="/facts/Solar_System/QIcLSazQ">Solar System</a>, as these bodies generally have orbits that lie close to the ecliptic.</li>
<li>The <a href="/facts/Equatorial_plane/P9Lyq2DE">equatorial plane</a> of the orbited body for satellites orbiting with small <a href="/facts/Semi-major_axis/l7JES2wo">semi-major axes</a></li>
<li>The local <a href="/facts/Laplace_plane/C6X80vMw">Laplace plane</a> for satellites orbiting with intermediate-to-large semi-major axes</li>
<li>The plane tangent to <a href="/facts/Celestial_sphere/TYAtXgor">celestial sphere</a> for extrasolar objects</li></ul>
On the plane of reference, a zero-point must be defined from which the angles of <a href="/facts/Longitude/HsC61ML9">longitude</a> are measured. This is usually defined as the point on the celestial sphere where the plane crosses the prime hour circle (the <a href="/facts/Hour_circle/bNIy6paq">hour circle</a> occupied by the <a href="/facts/First_Point_of_Aries/8bgH6zn4">First Point of Aries</a>), also known as the <a href="/facts/Equinox/s8kaFsD2">equinox</a>.

Orbital plane of reference open-in-new

Orbital plane of reference