Retrograde and prograde motion

Retrograde motion in astronomy is, in general, orbital or rotational motion of an object in the direction opposite the rotation of its primary, that is, the central object (right figure). It may also describe other motions such as precession or nutation of an object's rotational axis. Prograde or direct motion is more normal motion in the same direction as the primary rotates. However, "retrograde" and "prograde" can also refer to an object other than the primary if so described. The direction of rotation is determined by an inertial frame of reference, such as distant fixed stars. In the Solar System, the orbits around the Sun of all planets and most other objects, except many comets, are prograde. They orbit around the Sun in the same direction as the sun rotates about its axis, which is counterclockwise when observed from above the Sun's north pole. Except for Venus and Uranus, planetary rotations around their axis are also prograde. Most natural satellites have prograde orbits around their planets. Prograde satellites of Uranus orbit in the direction Uranus rotates, which is retrograde to the Sun. Nearly all regular satellites are tidally locked and thus have prograde rotation. Retrograde satellites are generally small and distant from their planets, except Neptune's satellite Triton, which is large and close. All retrograde satellites are thought to have formed separately before being captured by their planets. Most low-inclination artificial satellites of Earth have been placed in a prograde orbit, because in this situation less propellant is required to reach the orbit. When a galaxy or a planetary system forms, its material takes a shape similar to that 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. The nature of the collapse is explained by 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.

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