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clock drive
In astronomy a clock drive is a regulatory mechanism used to move an equatorial mounted telescope along one axis to keep the telescope in exact sync with the apparent motion of the celestial sky (diurnal motion).〔(Definition )〕
==Overview==
Clock drives work by rotating a telescope mount's polar axis, the axis parallel to the Earth's polar axis (also called the right ascension axis) in the opposite direction to the Earth's rotation one revolution every 23 hours and 56 minutes (called ''sidereal day''), thereby canceling that motion.〔(Turn left at Orion: a hundred night sky objects to see in a small telescope ... By Guy Consolmagno, Dan M. Davis, Karen Kotash Sepp, Anne Drogin, Mary Lynn Skirvin, page 204 )〕 This allows the telescope to stay fix on a certain point in the sky without having to be constantly re-aimed due to the Earth's rotation. The mechanism itself used to be clockwork but nowadays is usually electrically driven. Clock drives can be light and portable for smaller telescopes or can be exceedingly heavy and complex for larger ones such as the 60 inch telescope at the Mount Wilson Observatory.〔(60 inch clock ) the old Mount Wilson telescope clock drive〕 Clock-driven equatorial platforms are sometimes used in non-tracking type mounts, such as altazimuth mounts.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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