United States Naval Observatory - Wikipedia

Set my watch by it at least once a month! On August 31, 1842, the US Naval Observatory was authorized by an act of Congress. From the article:

President John Quincy Adams, who in 1825 signed the bill for the creation of a national observatory just before leaving presidential office, had intended for it to be called the National Observatory.[4] The names "National Observatory" and "Naval Observatory" were both used for 10 years, until a ruling was passed to officially use the latter.[5] Adams had made protracted efforts to bring astronomy to a national level at that time.[6][7] He spent many nights at the observatory, watching and charting the stars, which had always been one of Adams' avocations.

Established by the order of the United States Secretary of the Navy John Branch on 6 December 1830 as the Depot of Charts and Instruments,[8] the Observatory rose from humble beginnings. Placed under the command of Lieutenant Louis M. Goldsborough, with an annual budget of $330, its primary function was the restoration, repair, and rating of navigational instruments. It was made into a national observatory in 1842 via a federal law and a Congressional appropriation of $25,000. Lieutenant James Melville Gilliss was put in charge of "obtaining the instruments needed and books."[9] Lt. Gilliss visited the principal observatories of Europe with the mission to purchase telescopes and scientific devices and books.[10]

The observatory's primary mission was to care for the United States Navy's marine chronometers, charts, and other navigational equipment. It calibrated ships' chronometers by timing the transit of stars across the meridian. Opened in 1844 in Foggy Bottom north of the present site of the Lincoln Memorial and west of the White House (see: Old Naval Observatory), the observatory moved in 1893 to its present location[11] on a 2000-foot circle of land atop Observatory Hill overlooking Massachusetts Avenue. These facilities were listed on the National Register of Historic Places in 2017.[12]

The first superintendent was Navy Commander Matthew Fontaine Maury. Maury had the world's first vulcanized time ball, created to his specifications by Charles Goodyear for the U.S. Observatory. It was the first time ball in the United States, being placed into service in 1845, and the 12th in the world. Maury kept accurate time by the stars and planets. The time ball was dropped every day except Sunday precisely at the astronomically defined moment of Mean Solar Noon, enabling all ships and civilians to know the exact time. By the end of the American Civil War, the Observatory's clocks were linked via telegraph to ring the alarm bells in all of the Washington, D.C. firehouses three times a day, and by the early 1870s the Observatory's daily noon-time signal was being distributed nationwide via the Western Union Telegraph Company. Time was also "sold" to the railroads and was used in conjunction with railroad chronometers to schedule American rail transport. Early in the 20th century, the Arlington Time Signal broadcast this service to wireless receivers.

In 1849 the Nautical Almanac Office (NAO) was established in Cambridge, Massachusetts as a separate organization. It was moved to Washington, D.C. in 1866, colocating with the U. S. Naval Observatory in 1893. On September 20, 1894, the NAO became a "branch" of USNO, however it remained autonomous for several years after this.[13]

An early scientific duty assigned to the Observatory was the U.S. contribution to the definition of the Astronomical Unit, or the AU, which defines a standard mean distance between the Sun and the Earth, conducted under the auspices of the Congressionally funded U.S. Transit of Venus Commission. The astronomical measurements taken of the transit of Venus by a number of countries since 1639 resulted in a progressively more accurate definition of the AU. Relying heavily on photographic methods, the naval observers returned 350 photographic plates in 1874, and 1,380 measurable plates in 1882. The results of the surveys conducted simultaneously from several locations around the world (for each of the two transits) produced a final value of the solar parallax, after adjustments, of 8.809", with a probable error of 0.0059", yielding a U.S. defined Earth-Sun distance of 92,797,000 miles (149,342,000 km), with a probable error of 59,700 miles (96,100 km). This calculated distance was a significant improvement over several previous estimates.[14]

The telescope used for the discovery of the Moons of Mars was the 26-inch (66 cm) refractor (a telescope with a lens), then located at Foggy Bottom.[15] In 1893 it was moved to the present location.[16]

In November 1913 the Paris Observatory, using the Eiffel Tower as an antenna, exchanged sustained wireless (radio) signals with the United States Naval Observatory, using an antenna in Arlington, Virginia to determine the exact difference of longitude between the two institutions.[17]
In 1934, the last (then) large telescope to be installed at USNO saw "first light". This 40-inch aperture instrument[18] was also the second (and final) telescope made by famed optician, George Willis Ritchey. The Ritchey–Chrétien telescope design has since become the de facto optical design for nearly all major telescopes, including the famed Keck telescopes and the spaceborne Hubble Space Telescope. Unfortunately, light pollution forced USNO to think of other more viable locations to continue work, and so began a search. The final dark sky site chosen was Flagstaff, Arizona, and so the 40-inch telescope was moved to that location, beginning operations at the new Navy command, now called the Naval Observatory Flagstaff Station (NOFS). Those operations commenced in 1955,[19] and within a decade, the Navy's largest telescope, the 61-inch "Kaj Strand Astrometric Reflector" was built, seeing light at NOFS in 1964.[20]

The United States Naval Observatory no longer obtains significant astrometric observations, but it continues to be a major authority in the areas of Precise Time and Time Interval, Earth orientation, astrometry and celestial observation. In collaboration with many national and international scientific establishments, it determines the timing and astronomical data required for accurate navigation, astrometry, and fundamental astronomy and calculation methods — and distributes this information (such as star catalogs)[21] in the Astronomical Almanac, The Nautical Almanac, and on-line.[22]

Perhaps it is best known to the general public for its highly accurate ensemble of atomic clocks and its year 2000 time ball replacement.[23] The site also houses the largest astronomy library in the United States (and the largest astrophysical periodicals collection in the world).[24] The library includes a large collection of rare, often famous, physics and astronomy books from across the past millennium.

USNO continues to maintain its dark-sky observatory, NOFS, near Flagstaff, Arizona, which also now oversees the Navy Precision Optical Interferometer.[25] The Alternate Master Clock, mentioned above, also continues to operate at Schriever Air Force Base in Colorado.