The movement of a celestial body against the background of distant stars: When lying on the ground on a cloudless night, Sam and his girlfriend watched the apparent motion of the stars caused by the finite speed of light and the movement of the Earth while orbiting around the Sun.

The movement of an object, or celestial body, along a circular path (epicycle): 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 also called an epicycle motion, 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 150 A.D.

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

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: The proper motion of a star is due to the object's velocity relative to the Sun.

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 a planet in direct motion, the occurrence of retrograde motion usually indicates some peculiarity.

The motions developed by a robot element.