orbital inclination

An orbital inclination is the angle between the plane of an orbit and some presupposed or given reference plane. For example, the orbital inclination of an Earth satellite is generally given using the plane of the Earth's equator as a reference, but also could be given in reference to the plane of the Moon's orbit, which would likely be explicitly noted. The inclination of a solar system planet's orbit is generally given using the ecliptic (the plane of Earth's orbit) as a reference. The term is used for binary stars and extra-solar planets, generally in reference to the plane of the sky (the plane perpendicular to the line of sight). Examples:

• The Moon's orbit can be described by its inclination in relation to the ecliptic: this inclination varies slightly, averaging around 5.14°.
• The Moon's orbit can also be described by its inclination in relation to the plane of Earth's equator: this has a greater variation due to an 18-year precession cycle in the Moon's orbit (ranging between 23.45° and 28.6°).
• The Earth's orbit can be also described by its inclination in relation to the plane of the Sun's equator (7.155°).
• A transiting planet (exoplanet) has an orbital inclination close to 90° (in relation to the plane of the sky).