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Celestial Motions

Celestial objects exhibit two different types of motion. The first motion is such that the whole celestial sphere, and all of the celestial objects attached to it, rotates uniformly from east to west once every 24 (sidereal) hours, about a fixed axis passing through the earth's north and south poles. This type of motion is called diurnal motion, and is a consequence of the earth's daily rotation. Diurnal motion preserves the relative angular positions of all celestial objects. However, certain celestial objects, such as the sun, the moon, and the planets, possess a second motion, superimposed on the first, which causes their angular positions to slowly change relative to one another, and to the fixed stars. This intrinsic motion of objects in the solar system is due to a combination of the earth's orbital motion about the sun, and the orbital motions of the moon and the planets about the earth and the sun, respectively.

Richard Fitzpatrick 2010-07-21