# Clock drive > Mediated Wiki article. Canonical URL: https://mediated.wiki/source/Clock_drive > Markdown URL: https://mediated.wiki/source/Clock_drive.md > Source: https://en.wikipedia.org/wiki/Clock_drive > Source revision: 1245887034 > License: Creative Commons Attribution-ShareAlike 4.0 International (https://creativecommons.org/licenses/by-sa/4.0/) {{Short description|Mechanism in a telescope's mount}} {{citation style|date=November 2018}} [[File:Aldershot observatory 02.JPG|thumb|right|200px|Clock drive mechanism in the pier of the German equatorial mount for the 8-inch refracting telescope at [Aldershot Observatory](/source/Aldershot_Observatory).]] In [astronomy](/source/astronomy), a '''clock drive''' (also known as a '''sidereal drive''' or '''field rotator''') is a [motor-controlled](/source/motor_controller) mechanism used to move an [equatorial mount](/source/equatorial_mount)ed [telescope](/source/telescope) along one [axis](/source/axis_of_rotation) to keep the aim in exact sync with the [apparent motion](/source/diurnal_motion) of the [fixed stars](/source/fixed_stars) on the [celestial sphere](/source/celestial_sphere).[http://www.answers.com/topic/clock-drive Definition] ==Overview== Clock drives work by rotating a [telescope mount](/source/telescope_mount)'s polar axis, the axis parallel to the Earth's polar axis (also called the [right ascension](/source/right_ascension) axis) in the opposite direction to the Earth's rotation one revolution every 23 hours and 56 minutes (called ''[sidereal day](/source/Sidereal_time)''), thereby canceling that motion.[https://books.google.com/books?id=PexKTfPy3voC&dq=%22called+a+clock+drive%22&pg=PA204 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 fixed 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](/source/clockwork) but nowadays is usually electrically driven. Clock drives can be light and portable for smaller telescopes{{cite web |last=Oltion |first=Jerry |title=The Trackball Mount |url=http://www.sff.net/people/j.oltion/trackball_mount.htm |access-date=13 March 2011}} or can be exceedingly heavy and complex for larger ones such as the 60-inch telescope at the [Mount Wilson Observatory](/source/Mount_Wilson_Observatory).[http://www.oldengine.org/members/levans/60clock/ 60 inch clock] {{webarchive|url=https://web.archive.org/web/20110727142221/http://www.oldengine.org/members/levans/60clock/ |date=2011-07-27 }} the old Mount Wilson telescope clock drive Clock-driven [equatorial platform](/source/equatorial_platform)s are sometimes used in non-tracking type mounts, such as [altazimuth mount](/source/altazimuth_mount)s.{{cite web |last=Vogel |first=Reiner |title=Circle Segment Platform |language=English |year=2007 |url=http://www.reinervogel.net/index_e.html |access-date=13 March 2011}} ==History== The original inventor of the clock drive is unknown, being an idea that was tried in several ways throughout history.[https://books.google.com/books?id=KAWwzHlDVksC&dq=clock+drive+king&pg=PR8 Henry C. King, The History of the Telescope, page viii] In [China](/source/China) in 1094, during the [Song dynasty](/source/Song_dynasty), [Su Song](/source/Su_Song) built a water-driven [clock tower](/source/clock_tower) that had many features, including a 20-ton bronze [armillary sphere](/source/armillary_sphere) that stayed in alignment with the heavens.Joseph Needham, Clerks and Craftsmen in China and the West, page 130 Different types of equatorial clock-driven telescopes were built or proposed in the 17th and 18th century including English astronomer [Robert Hooke](/source/Robert_Hooke)'s 1674 paper proposing their use in precision measurement,Allan Chapman, England's Leonardo: Robert Hooke and the Seventeenth-Century, page 169 a clock-driven [aerial telescope](/source/aerial_telescope) objective lens constructed in 1685 by Italian-French astronomer [Giovanni Domenico Cassini](/source/Giovanni_Domenico_Cassini) fitted with [setting circles](/source/setting_circles),Henry C. King, The History of the Telescope, page 61 and one supposedly built by English clockmaker and inventor [George Graham](/source/George_Graham_(clockmaker)) early 18th century.[Henry Lonsdale](/source/Henry_Lonsdale), George Routledge & sons, ''The Worthies of Cumberland'', 1875, page 36 What is considered the first practical clock-driven telescope was constructed in 1824, [Joseph von Fraunhofer](/source/Joseph_von_Fraunhofer)'s ‘[Great Dorpat](/source/Tartu_Observatory)’ refractor, a telescope that combined other innovations, such as an [achromatic lens](/source/achromatic_lens) and Fraunhofer's "[German equatorial mount](/source/Equatorial_mount)", making it the prototype of all future large [refracting telescope](/source/refracting_telescope)s.[http://www.artdeciel.com/telescopes_detail.aspx?Telescope_ID=36 artdeciel.com - Dorpat 9.6-inch Refractor] {{webarchive|url=https://web.archive.org/web/20100924063022/http://artdeciel.com/telescopes_detail.aspx?Telescope_ID=36 |date=2010-09-24 }}Sam Brown, All about telescopes, 1976, 192 pagesBryan H. Bunch, Alexander Hellemans, The History of Science and Technology: A Browser's Guide, page 324 Early telescope clock drives used a clock work with falling weights and pendulums, almost like grandfather clocks.{{cite web | url=http://www.wisegeek.com/what-is-a-clock-drive.htm | title=What is a Clock Drive? (With pictures) }} Over time the falling weights and pendulums were replaced by an electric motor. ==See also== * [GoTo (telescopes)](/source/GoTo_(telescopes)) *[Guide star](/source/Guide_star) * [Polar alignment](/source/Polar_alignment) * [Setting circles](/source/Setting_circles) ==Notes== {{Reflist}} Category:Optical telescope components --- Adapted from the Wikipedia article [Clock drive](https://en.wikipedia.org/wiki/Clock_drive) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Clock_drive?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.