Inclination in general is the angle between a reference plane and another plane or axis of direction.
The inclination is one of the six orbital parameters describing the shape and orientation of a celestial orbit. It is the angular distance of the orbital plane from the plane of reference (usually the primary's equator or the ecliptic), normally stated in degrees.
In the solar system, the inclination of the orbit of a planet is defined as the angle between the plane of the orbit of the planet and the ecliptic — which is the plane containing Earth's orbital path. It could be measured with respect to another plane, such as the Sun's equator or even Jupiter's orbital plane, but the ecliptic is more practical for Earth-bound observers. Most planetary orbits in our solar system have relatively small inclinations, both in relation to each other and to the Sun's equator. There are notable exceptions in the dwarf planets Pluto and Eris, which have inclinations to the ecliptic of 17 degrees and 44 degrees respectively, and the large asteroid Pallas, which is inclined at 34 degrees.
Natural and artificial satellites
The inclination of orbits of natural or artificial satellites is measured relative to the equatorial plane of the body they orbit if they do so close enough. The equatorial plane is the plane perpendicular to the axis of rotation of the central body.
- an inclination of 0 degrees means the orbiting body orbits the planet in its equatorial plane, in the same direction as the planet rotates;
- an inclination greater than -90° and less than 90° is a prograde orbit.
- an inclination greater than 90° and less than 270° is a retrograde orbit.
- an inclination of exactly 90° is a polar orbit, in which the spacecraft passes over the north and south poles of the planet; and
- an inclination of exactly 180° is a retrograde equatorial orbit.
- an inclination of exactly 270° is a polar orbit in the opposite direction to a 90° polar orbit.
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