A transit telescope is a special purpose telescope mounted so as to allow it to be pointed only at objects in the sky crossing the local meridian, an event known as a transit. These telescopes rely on the rotation of the Earth to bring objects into their field of view.
Transit telescopes have been used since the 18th century to accurately measure positions of stars in order to catalogue them. This is done by measuring the instant when the star passes through the local meridian. Its altitude above the horizon is noted as well. Knowing one's geographic latitude and longitude these measurements can be used to derive the star's right ascension and declination.
Once good star catalogues were available a transit telescope could be used anywhere in the world to accurately measure local longitude and time by observing local meridian transit times of catalogue stars. Prior to the invention of the atomic clock this was the most reliable source of accurate time.
A modern day example of this type of telescope is the 0.2m Flagstaff Astrometric Scanning Transit Telescope (FASTT).
Vision for the Future : Scots Artists across the World Are Donating Works to Help Edinburgh's Collective Gallery Widen Its Horizons with a New Home in the City Observatory
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