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diurnal circle(dīûr`nəl), apparent path followed by a star due to the earth's rotation on its axis. The stars appear to move on the celestial spherecelestial sphere,
imaginary sphere of infinite radius with the earth at its center. It is used for describing the positions and motions of stars and other objects. For these purposes, any astronomical object can be thought of as being located at the point where the line of sight
..... Click the link for more information. 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 starscircumpolar star,
star whose diurnal circle lies completely above or completely below an observer's horizon. A star whose diurnal circle lies above the horizon never sets, even though it cannot be seen during the day.
..... Click the link for more information. .
diurnal circleSee diurnal motion.
a small circle of the celestial sphere in a plane parallel to the plane of the celestial equator. Diurnal circles are paths along which points of the celestial sphere (other than the celestial poles) travel owing to the apparent diurnal rotation of the celestial sphere about the celestial axis.