Improper motion
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The improper motion of a star refers to the change of its coordinates on the sky not originating from the motion of the star itself, as opposed to proper motion. Obvious sources of improper motion include diurnal motion (caused by the rotation of the Earth) and parallax (caused by the Earth's orbit around the sun). However, nutation and precession of the equinoxes must also be taken into account. Finally, aberration of light itself can cause apparent motion.
The term "improper" derives from an older use of "proper" meaning "belonging to" (cf, propre in French).[1]
