diurnal circle (dĪûrˈnəl) [key], apparent path followed by a star due to the earth's rotation on its axis. The stars appear to move on the celestial sphere in concentric circular paths centered at the celestial poles. Since the earth rotates west to east, the stars appear to move from east to west along their diurnal circles. Stars whose diurnal circles lie completely above the horizon are called circumpolar stars.
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