This course will explore the phenomenon of planetary retrograde motion, which is the apparent backward motion of planets in the night sky. We will discuss the causes of this phenomenon, including the orbital motion of the planets, the Earth's motion around the Sun, and the effects of gravitational forces. We
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. 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.