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|Description||The 6-inch Transit Circle at USNO was built in 1898 by Warner & Swasey, and was used continuously from then until 1995. It is a highly specialized instrument, rigidly mounted so that it can only look along a line passing from north to south through the zenith (the point directly overhead). A star's position is measured as it crosses, or "transits", a set of crosshairs mounted at the telescope's focal plane. The time of the star's transit, measured against a celestial reference frame, gives its "Right Ascension" or celestial longitude. The star's altitude is measured directly at the telescope, where two circles, each divided into 7200 sectors, are read by the observer. The star's altitude can then be converted directly into its "Declination", or celestial latitude. Transit Circles could determine stellar positions to accuracies approaching 0.05 arcseconds.|
|Author||US Naval Observatory|
|This image is a work of a U.S. military or Department of Defense employee, taken or made during the course of an employee's official duties. As a work of the U.S. federal government, the image is in the public domain.
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