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The 6-inch Warner & Swasey Transit Circle

Transit Circle telescopes are no longer actively used by the USNO in Washington, DC for astrometric observations, but until quite recently they were the backbone instruments for measuring precise star positions and the determination of time.

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.

Although these measurements are no longer carried out today, their legacy forms the basis of many of our star catalogs. The telescope celebrated the centennial of its first observation on 14 February, 1998. The final observation with the instrument was made on the following day. During its long career nearly one million individual star position measurements were made. It has recently been erected as a permanent historical display in the lobby of the USNO's main building.

Visual Transit Circle telescopes are being replaced by newer instruments capable of determining stellar positions to an accuracy of 0.01 arcseconds. USNO operates an automated scanning transit telescope at its Flagstaff Station, and is developing the Navy Prototype Optical Interferometer (NPOI). USNO is also using a Very Long Baseline Interferometer (VLBI) array of radio telescopes to measure the precise positions of distant quasars. The positions of these objects form a measurement frame precise enough to measure the small oscillations of the Earth's rotational poles!

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