The movement of a celestial body against the background of distant stars.

According to the geocentric Ptolemaic system, planetary orbits have two components: a circular or deferent orbit around the parent body; and a smaller circular orbit, or epicycle, around a point on the deferent orbit.

The deferent is the large circular orbit around which a planet was thought to orbit, in one or many epicycles.

Epicycles are circular orbits within orbits that were used to (incorrectly) describe the orbits of objects in the Ptolemaic system (about A.D. 150).

Epicycles and deferents were used to predict orbits until Kepler discovered the elliptical nature of orbits (early in the 1600's).

Passively drifting and wandering in the sky unit.

The movement of a star in the celestial sphere which is more noticeable in near stars than in distant stars; in practice, the movement of the latter is negligible.

1. The apparent backward movement, or reversal of direction, by outer planets in the solar system.

Retrograde motion is simply an optical illusion, created by the fact that the earth is orbiting the sun much faster than the outer planets are.

2. The clockwise, or east to west, motion of a body, and hence the reverse of direct motion.

As the majority of bodies in the solar-system orbit around their governing bodies; that is, the sun or planet, in direct motion; the occurrence of retrograde motion usually indicates some peculiarity.

The motions developed by a robot element.