Orbital plane (astronomy)
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The orbital plane of an object orbiting another is the geometrical plane in which the orbit is embedded. Three points are required to find the orbital plane: the center of the heavier object, the center of the orbiting object and the center of the orbiting object at some later time.
By definition the inclination of a planet in the solar system is the angle between its orbital plane and that of the Earth. In other cases, for instance a moon orbiting another planet, it is convenient to define the inclination of the moon's orbit as the angle between its orbital plane and the planet's equator.
[edit] Artificial satellites around the Earth
For launch vehicles and artificial satellites, the orbital plane is a defining parameter of an orbit; as in general, it will take a very large amount of propellant to change the orbital plane of an object. Other parameters, such as the orbital period, the eccentricity of the orbit and the phase of the orbit are more easily changed by propulsion systems.
Orbital planes of satellites are perturbed by the non-spherical nature of the Earth's gravity, and this causes the orbital plane to slowly rotate around the Earth, depending on the angle of the plane. For planes that are at a critical angle this can mean that the plane will track the Sun around the Earth, forming a Sun synchronous orbit.
Launch vehicle's launch windows are usually determined by when the points in the time that the target orbital plane intersects the launch site.
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