The six-inch Warner & Swasey Transit Circle, in use 1899 - 1999
Transit Circle telescopes are no longer actively used by the USNO in Washington, DC for astrometric observations, but throughout most of the 19th and 20th Centuries they were the backbone instruments for measuring precise star positions and the determination of time at observatories around the world.
The 6-inch Transit Circle at USNO was designed by USNO astronomer Prof. William Harkness and built in 1898 by the Warner & Swasey Company of Cleveland, Ohio, and was used continuously from 1899 until 1995. Occasional observations to verify certain bright star positions continued for several subsequent years. Installed in the "west transit house", It was a highly specialized instrument, rigidly mounted so that it could only look along the local meridian line, which passes from north to south through the zenith (the point directly overhead). It was the first instrument of its kind to be constructed entirely out of steel.
A star's position is measured as it crosses, or "transits", a set of fixed wire 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.
During the 20th Century the 6-inch instrument received many upgrades. The visual microscopes used to read the divided altitude circles were replaced, first with film cameras in the 1940s, then CCD digital cameras in the 1980s. The fixed micrometer wires were superseded by a moving-wire device in the 1930s.
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, 1999. Its final recorded observation was made on the following day. During its long career over 938,000 individual star position measurements were made. It has been erected as a permanent historical display in the lobby of the USNO's main building. All told, USNO transit circles made 1,878,153 measurements going back to 1866.
Visual Transit Circle telescopes have been replaced by newer instruments capable of determining stellar positions to an accuracy of 0.01 arcseconds. USNO operates an 8-inch automated scanning transit telescope at its Flagstaff Station, and is operating the Navy Precision Optical Interferometer (NPOI) in collaboration with Lowell Observatory and the Naval Research Laboratory.