You are here: Home USNO James M. Gilliss Library Historical Photos, Artwork, Objects Historical Collection Telescopes Six-Inch Transit Circle Telescope

Six-Inch Transit Circle Telescope

The 6-Inch Transit Circle Telescope was the workhorse instrument at the Naval Observatory for most of the 20th century. It was used to observe precise positions of the Sun, Moon, planets and stars, and thereby defined the fundamental celestial coordinate system. The six-inch transit circle is so named because its lens is six inches in diameter, and because the east-west coordinate (the right ascension) of a star was measured by timing when it crossed, or "transited" the meridian due to the Earth's rotation. The north-south coordinate (the declination) is measured with a very finely divided circle with 7,200 divisions. In both coordinates precise measurements are made with respect to a system of wires in the micrometer in the focal plane of the eyepiece. Warner and Swasey of Cleveland, Ohio built the telescope in 1897 in accordance with the specifications of William Harkness. It was mounted in December, 1897 in the West transit house, and was used beginning in June, 1899. In order to reduce the effect of flexure and temperature changes the instrument was built entirely of steel. The telescope remained in use until 1995, when it was superseded by new technologies.
USNO Master Clock Time
Javascript must be Enabled