A US government observatory in Washington, DC, the main purpose of which is to provide the astronomical data required to support Navy and other Department of Defense activities. These include astrometry, the preparation of almanacs, time measurement and the maintenance of the Master Clock for the USA. It has astrographic telescopes located at Anderson Mesa, near Flagstaff, Arizona, and Black Birch, New Zealand, as well as in Washington. The observatory was founded in 1830 and given the title US Naval Observatory in 1844. For fifty years it was located at the site now occupied by the Lincoln Memorial. It was moved to its present site, next to the official residence of the Vice President, in 1893. The largest telescope at the site is the 66-centimetre 26-inch refractor, dating from 1873, with which Asaph Hall discovered the moons of Mars, Phobos and Deimos, in 1877. Other instruments there include a 30-centimetre 12-inch Alvan Clark refractor, two 61-centimetre 24-inch reflectors and a 15-centimetre 6-inch transit circle. The largest telescope belonging to the observatory is the 1.5-metre 61-inch astrometric reflector at Flagstaff. Using this instrument, James Christy discovered the moon of Pluto, Charon, in 1978. At the Arizona site, the observatory has also constructed an optical interferometer, the Navy Prototype Optical Interferometer, which was the largest of its kind when it came into operation in 1995. The US Naval Observatory also houses one of the world’s leading astronomical libraries.