What is the United States Naval Observatory?


The United States Naval Observatory (USNO) is one of the oldest scientific agencies in the United States, with a primary mission to produce positioning, navigation and timing for the United States Navy and the United States Department of Defence.

The Seal of the USNO with a quote from the Astronomica by Marcus Manilius, Adde gubernandi studium: Pervenit in astra, et pontum caelo conjunxit [Increase the study of navigation: It arrives in the stars, and marries the sea with heaven.]

The observatory is located in Northwest Washington, D.C. at the Northwestern end of Embassy Row. It is one of the pre-1900 astronomical observatories located in an urban area; at the time of its construction, it was far from the light pollution thrown off by the (then-smaller) city centre.

Former USNO director Gernot M. R. Winkler initiated the “Master Clock” service that the USNO still operates, and which provides precise time to the GPS satellite constellation run by the United States Space Force. The USNO performs radio VLBI-based positions of quasars for astrometry and geodesy with numerous global collaborators (IERS), in order to produce Earth orientation parameters and to realize the celestial reference system (ICRF).

Aside from its scientific mission, a house located within the Naval Observatory campus serves as the official residence of the vice president of the United States.

Brief History

Early Presidential Astronomical Interest

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.

The names “National Observatory” and “Naval Observatory” were both used for 10 years, until a ruling was passed to officially use the latter.

Adams had made protracted efforts to bring astronomy to a national level at that time. He spent many nights at the observatory, watching and charting the stars, which had always been one of Adams’ interests.

Establishment as an Optical Equipment Depot

Established by order of the United States Secretary of the Navy John Branch on 06 December 1830 as the Depot of Charts and Instruments, 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.

Federal Observatory

It was established as a national observatory in 1842 by federal law and a Congressional appropriation of $25,000. Lieutenant J.M. Gilliss was put in charge of “obtaining the instruments needed and books.” Lieutenant Gilliss visited the principal observatories of Europe with the mission to purchase telescopes and other scientific devices, and books.

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. It 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 to its present location in 1893 located on a 2000 foot circle of land atop “Observatory Hill”, overlooking Massachusetts Avenue.

The facilities were listed on the National Register of Historic Places in 2017.

The Iconic Time-Ball

The first superintendent was Navy Commander M.F. Maury. Maury had the world’s first vulcanized time ball, created to his specifications by Charles Goodyear for the US Observatory. Placed into service in 1845, it was the first time ball in the United States 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; this enabled all ships and civilians within sight 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.

The Nautical Almanac Office

In 1849 the Nautical Almanac Office (NAO) was established in Cambridge, Massachusetts as a separate organisation. It was moved to Washington, D.C. in 1866, originally operating near Fort Myer. It relocated to the US Naval Observatory grounds in 1893.

On 20 September 1894, the NAO became a “branch” of USNO; however, it remained autonomous for several years afterward.

Measuring the Astronomical Unit

An early scientific duty assigned to the Observatory was the US contribution to the definition of the Astronomical Unit, or the au, which defines a standard mean distance between the Sun and the Earth. This was conducted under the auspices of the congressionally-funded US 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 strongly 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 US-determined Earth-Sun distance of 92,797,000 miles (149,342,000 km), with a probable error of 59,700 miles (96,100 km). The calculated distance was a significant improvement over several previous estimates.

The New 26 Inch and 40 Inch Refractors

The telescope used for the discovery of the Moons of Mars was the 26 inch (66 cm) refractor (a telescope containing only lenses), then located at Foggy Bottom, Washington, DC. In 1893 it was moved to the present location.

In 1934, the last (then) large telescope to be installed at USNO saw “first light”. This 40 inch aperture instrument 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 space-borne Hubble Space Telescope.

Because of light pollution in the Washington metropolitan area, USNO relocated the 40 inch telescope to Flagstaff, Arizona. A new Navy command, now called the USNO Flagstaff Station (NOFS), was established there. Those operations began in 1955. Within a decade, the Navy’s largest telescope, the 61 inch “Kaj Strand Astrometric Reflector” was built; it saw light at Flagstaff in 1964.

USNO continues to maintain its dark-sky observatory, NOFS, near Flagstaff. This facility now oversees the Navy Precision Optical Interferometer. The alternative Master Clock, discussed below, continues to operate at Schriever Air Force Base in Colorado.

Brief History of the Time Service

By the early 1870s the USNO daily noon-time signal was distributed electrically, 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 service was broadcast by radio, with Arlington time signal available to those with wireless receivers.

In November 1913 the Paris Observatory, using the Eiffel Tower as an antenna, exchanged sustained wireless (radio) signals with the US Naval Observatory to determine the exact difference of longitude between the two institutions, via an antenna in Arlington, Virginia.

The US Naval Observatory in Washington 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 catalogues) on-line and in the annual publications The Astronomical Almanac and The Nautical Almanac.

The general public knows about the “Master Clock”, the observatory’s highly accurate ensemble of atomic clocks, and its one-off time-ball re-enactment for the year-2000 celebration. The site houses the largest astronomy library in the United States (and the largest astrophysical periodicals collection in the world). The library includes a large collection of rare, often famous, physics and astronomy books from the past millennium.

Aerial view of the US Naval Observatory Washington, DC, campus.


In 1990 two departments were established: Orbital Mechanics and Astronomical Applications, with the Nautical Almanac Office a division in Astronomical Applications. The Orbital Mechanics Department operated under P. Kenneth Seidelmann until 1994, when the department was abolished and its functions transferred to a group within the Astronomical Applications Department.

In 2010, USNO’s astronomical ‘department’ known as the Naval Observatory Flagstaff Station (NOFS) was officially made autonomous as an Echelon 5 command, separate from, but still reporting to the USNO in Washington. In the alpine woodlands above 7,000 feet altitude outside Flagstaff, Arizona, NOFS performs its national, Celestial Reference Frame (CRF) mission under dark skies in that region.

Official Residence of the Vice President of the United States

A house situated on the grounds of the observatory, at Number One Observatory Circle, has been the official residence of the vice president of the United States since 1974. It is protected by tight security control enforced by the Secret Service. The house is separated from the Naval Observatory.

It formerly served as the residence of the observatory’s superintendent, and later was the residence of the chief of naval operations, and finally the vice president, in a series of usurpations which sent the former resident packing.

Time Service

The U.S. Naval Observatory operates two “Master Clock” facilities, one in Washington, DC, and the other at Schriever AFB near Colorado Springs, CO.

  • The primary facility, in Washington, D.C. maintains 57 HP/Agilent/Symmetricom 5071A-001 high performance cesium atomic clocks and 24 hydrogen masers.
  • The alternate facility, at Schriever Air Force Base, maintains 12 cesium clocks and 3 masers.

The observatory also operates four rubidium atomic fountain clocks, which have a stability reaching 7×10-16. The observatory plans to build several more of this type for use at its two facilities.

The clocks used for the USNO timescale are kept in 19 environmental chambers, whose temperatures are kept constant to within 0.1°C. The relative humidity is kept constant in all maser, and most cesiums enclosures, to within 1%. Time-scale management only uses the clocks in Washington, DC, and of those, preferentially uses the clocks that currently conform reliably to the time reports of the majority. It is the combined ‘vote’ of the ensemble that constitutes the otherwise-fictitious “Master Clock”. The time-scale computations on 07 June 2007 weighted 70 of the clocks into the standard.

The US Naval Observatory provides public time service via 26 NTP servers on the public Internet, and via telephone voice announcements:

  • +1 202 762-1401 (Washington, DC).
  • +1 202 762-1069 (Washington, DC).
  • +1 719 567-6742 (Colorado Springs, CO).

The voice of actor Fred Covington (1928-1993) has been announcing the USNO time since 1978.The voice announcements always begin with the local time (daylight or standard), and include a background of 1 second ticks. Local time announcements are made on the minute, and 15, 30, and 45 seconds after the minute. Coordinated Universal Time (UTC) is announced 5 seconds after the local time. Upon connecting, only the second-marking ticks are heard for the few seconds before the next scheduled local time announcement.

The USNO also operates a modem time service, and provides time to the Global Positioning System (GPS).

Instrument Shop

The United States Naval Observatory Instrument shop has been designing and manufacturing precise instrumentation since the early 1900s.


  • Astronomical Observations made at the US Naval Observatory (USNOA) (v. 1-6: 1846-1867).
  • Astronomical and Meteorological Observations made at the US Naval Observatory (USNOM) (v. 1-22: 1862-1880).
  • Observations made at the US Naval Observatory (USNOO) (v. 1-7: 1887-1893).
  • Publications of the US Naval Observatory, Second Series (PUSNO) (v. 1-16: 1900-1949).
  • US Naval Observatory Circulars.
  • The Astronomical Almanac.
  • The Nautical Almanac.
  • The Air Almanac.
  • Astronomical Phenomena.

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