retrograde motion
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Retrograde motion is when an astronomical body orbits or rotates in the opposite direction compared to the body it circles around. This matters because it's unusual and can tell us important things about how planetary systems formed and evolved.
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Retrograde orbit: the satellite (red) orbits in the direction opposite to the rotation of its primary (blue/black)
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.