Retrograde and direct motion

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Retrograde motion is in the direction opposite to the movement of something else, and is the contrary of direct or prograde motion. This motion can refer to the orbit of one body about another body or about some other point, or to the rotation of a single body about its axis, or other orbital parameters such as precession or nutation of the axis. In reference to celestial systems, retrograde motion usually means motion which is contrary to the rotation of the primary, that is, the object which forms the system's hub.

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Formation of celestial systems

When a galaxy or a planetary system forms, its material takes the shape of a disk. Most of the material orbits and rotates in one direction. This uniformity of motion is due to the collapse of a gas cloud.[1] The nature of the collapse is explained by the principle called conservation of angular momentum. In 2010 the discovery of several hot jupiters with backward orbits called into question the theories about the formation of planetary systems.[2]

Inclination

A celestial object's inclination indicates whether the object's orbit is direct or retrograde. The inclination of a celestial object is the angle between its orbital plane and another reference frame such as the equatorial plane of the object's primary. In our solar system, inclination of the planets is often measured from the ecliptic plane, which is the plane of Earth's orbit around the Sun.[3] The inclination of moons is measured from the equator of the planet they orbit. An object with an inclination between -90 and +90 degrees is orbiting or revolving in the same direction as the primary is rotating. An object with an inclination of exactly 90 degrees has a perpendicular orbit which is neither direct nor retrograde. An object with an inclination beyond 90 degrees up to 270 degrees is in a retrograde orbit.

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