United States Naval Observatory

Last updated
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.] U.S. Naval Observatory-seal.gif
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.]

TheUnited States Naval Observatory (USNO) is a scientific and military facility that produces geopositioning, navigation and timekeeping data for the United States Navy and the United States Department of Defense. [1] Established in 1830 as the Depot of Charts and Instruments, it is one of the oldest scientific agencies in the United States, [2] and remains the country's leading authority for astronomical and timing data for all purposes. [3]


The observatory is located in Northwest Washington, D.C. at the northwestern end of Embassy Row. It is among the few pre-20th century astronomical observatories located in an urban area; initially located in Foggy Bottom near the city's center, it was relocated to its current location in 1893 to escape light pollution.

The USNO has conducted significant scientific studies throughout its history, including measuring the speed of light, observing solar eclipses, and discovering the moons of Mars. [4] Its achievements including providing data for the first radio time signals, constructing some of the earliest and most accurate telescopes of their kind, and helping develop universal time. [3] The Naval Observatory 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, the Naval Observatory campus hosts the official residence of the vice president of the United States.


Aerial view of the U.S. Naval Observatory Washington, DC, campus United States Naval Observatory.aerial view.jpg
Aerial view of the U.S. Naval Observatory Washington, DC, campus
The 26 inch (66 cm) aperture telescope, with which Asaph Hall discovered the moons of Mars in 1877; the telescope is shown here at its new location. Usno-telescope-equalized-1.png
The 26 inch (66 cm) aperture telescope, with which Asaph Hall discovered the moons of Mars in 1877; the telescope is shown here at its new location.

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. [5]

The names "National Observatory" and "Naval Observatory" were both used for 10 years, until the Secretary of the Navy officially adopted the latter. [6]

Adams had made protracted efforts to bring astronomy to a national level at that time. [7] [8] 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 6 December 1830 as the Depot of Charts and Instruments, [9] 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. Lt.  J.M. Gilliss was put in charge of "obtaining the instruments needed and books." [10] Lt. Gilliss visited the principal observatories of Europe with the mission to purchase telescopes and other scientific devices, and books. [11]

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 [12] 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. [13]

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 U.S. 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 organization. It was moved to Washington, D.C. in 1866, originally operating near Fort Myer. It relocated to the U.S. Naval Observatory grounds in 1893. [14]

On 20 September 1894, the NAO became a "branch" of USNO; however, it remained autonomous for several years afterward. [14]

Measuring the astonomical unit

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. This was 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 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 U.S.-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. [15]

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. [16] In 1893 it was moved to the present location. [17]

Naval Observatory Flagstaff Station NOFS panorama sm.jpg
Naval Observatory Flagstaff Station

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 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. [19] Within a decade, the Navy's largest telescope, the 61 inch "Kaj Strand Astrometric Reflector" was built; it saw light at Flagstaff in 1964. [20]

USNO continues to maintain its dark-sky observatory, NOFS, near Flagstaff. This facility now oversees the Navy Precision Optical Interferometer. [21] The alternate Master Clock, discussed below, continues to operate at Schriever Air Force Base in Colorado.

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”[ citation needed ] 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 U.S. Naval Observatory to determine the exact difference of longitude between the two institutions, via an antenna in Arlington, Virginia. [22]

The U.S. 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 catalogs) [23] on-line and in the annual publications The Astronomical Almanac and The Nautical Almanac . [24]

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. [25] The site houses the largest astronomy library in the United States (and the largest astrophysical periodicals collection in the world). [26] The library includes a large collection of rare, often famous, physics and astronomy books from the past millennium.

Former USNO director Gernot M. R. Winkler initiated the "Master Clock" service that the USNO still operates, [27] and which provides precise time to the GPS satellite constellation run by the United States Space Force.


In 1990 two departments were established: Orbital Mechanics and Astronomical Applications, with the Nautical Almanac Office a division in Astronomical Applications. [14] [28] 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. [14]

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

Number One Observatory Circle, official home of the U.S. vice president. 1OC2003.jpg
Number One Observatory Circle, official home of the U.S. vice president.

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. [29]

Time service

Atomic clock ensemble at the U.S. Naval Observatory Usno-mc.jpg
Atomic clock ensemble at the U.S. Naval Observatory

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

The observatory also operates four [31] rubidium atomic fountain clocks, which have a stability reaching 7×10−16. [32] The observatory plans to build several more of this type for use at its two facilities. [30]

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 humidities are 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 7 June 2007 weighted 70 of the clocks into the standard. [30]

The U.S. Naval Observatory provides public time service via 26  NTP [30] servers on the public Internet, [33] and via telephone voice announcements: [34]

The voice of actor Fred Covington (1928–1993) [35] has been announcing the USNO time since 1978. [36]

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. [37] 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, [38] and provides time to the Global Positioning System.

Instrument shop

The United States Naval Observatory Instrument shop has been designing and manufacturing precise instrumentation since the early 1900s. [39]


Navy Precision Optical Interferometer, Flagstaff, Arizona NPOI.Usic.jpg
Navy Precision Optical Interferometer, Flagstaff, Arizona

See also

Astronomy and observatories

Technology and technical resources

USNO personnel


    Related Research Articles

    <span class="mw-page-title-main">Astrometry</span> Branch of astronomy involving positioning and movements of celestial bodies

    Astrometry is a branch of astronomy that involves precise measurements of the positions and movements of stars and other celestial bodies. It provides the kinematics and physical origin of the Solar System and this galaxy, the Milky Way.

    <span class="mw-page-title-main">Star catalogue</span> Astronomical catalogue that lists stars and their positions in the sky

    A star catalogue is an astronomical catalogue that lists stars. In astronomy, many stars are referred to simply by catalogue numbers. There are a great many different star catalogues which have been produced for different purposes over the years, and this article covers only some of the more frequently quoted ones. Star catalogues were compiled by many different ancient people, including the Babylonians, Greeks, Chinese, Persians, and Arabs. They were sometimes accompanied by a star chart for illustration. Most modern catalogues are available in electronic format and can be freely downloaded from space agencies' data centres. The largest is being compiled from the spacecraft Gaia and thus far has over a billion stars.

    <span class="mw-page-title-main">Department of Defense master clock</span> Atomic master clock for the U.S. Department of Defense

    The Department of Defense master clock is the atomic master clock to which time and frequency measurements for the United States Department of Defense are referenced.

    <span class="mw-page-title-main">Time standard</span> Specification for measuring time

    A time standard is a specification for measuring time: either the rate at which time passes or points in time or both. In modern times, several time specifications have been officially recognized as standards, where formerly they were matters of custom and practice. An example of a kind of time standard can be a time scale, specifying a method for measuring divisions of time. A standard for civil time can specify both time intervals and time-of-day.

    <span class="mw-page-title-main">Lowell Observatory</span> Astronomical observatory and historic landmark in Arizona, US

    Lowell Observatory is an astronomical observatory in Flagstaff, Arizona, United States. Lowell Observatory was established in 1894, placing it among the oldest observatories in the United States, and was designated a National Historic Landmark in 1965. In 2011, the Observatory was named one of "The World's 100 Most Important Places" by Time Magazine. It was at the Lowell Observatory that the dwarf planet Pluto was discovered in 1930 by Clyde Tombaugh.

    <span class="mw-page-title-main">Asaph Hall</span> American astronomer (1829–1907)

    Asaph Hall III was an American astronomer who is best known for having discovered the two moons of Mars, Deimos and Phobos, in 1877. He determined the orbits of satellites of other planets and of double stars, the rotation of Saturn, and the mass of Mars.

    <span class="mw-page-title-main">Matthew Fontaine Maury</span> Navy officer

    Matthew Fontaine Maury was an American oceanographer and naval officer, serving the United States and then joining the Confederacy during the American Civil War.

    <span class="mw-page-title-main">Nautical almanac</span>

    A nautical almanac is a publication describing the positions of a selection of celestial bodies for the purpose of enabling navigators to use celestial navigation to determine the position of their ship while at sea. The Almanac specifies for each whole hour of the year the position on the Earth's surface at which the Sun, Moon, planets, and First Point of Aries is directly overhead. The positions of 57 selected stars are specified relative to the First Point of Aries.

    <span class="mw-page-title-main">Navy Precision Optical Interferometer</span> US Navy astronomical interferometer

    The Navy Precision Optical Interferometer (NPOI) is an American astronomical interferometer, with the world's largest baselines, operated by the Naval Observatory Flagstaff Station (NOFS) in collaboration with the Naval Research Laboratory (NRL) and Lowell Observatory. The NPOI primarily produces space imagery and astrometry, the latter a major component required for the safe position and navigation of all manner of vehicles for the DoD. The facility is located at Lowell's Anderson Mesa Station on Anderson Mesa about 25 kilometers (16 mi) southeast of Flagstaff, Arizona (US). Until November 2011, the facility was known as the Navy Prototype Optical Interferometer (NPOI). Subsequently, the instrument was temporarily renamed the Navy Optical Interferometer, and now permanently, the Kenneth J. Johnston Navy Precision Optical Interferometer (NPOI) – reflecting both the operational maturity of the facility, and paying tribute to its principal driver and retired founder, Kenneth J. Johnston.

    <span class="mw-page-title-main">McCormick Observatory</span> Observatory

    The Leander McCormick Observatory is one of the astronomical observatories operated by the Department of Astronomy of the University of Virginia, and is situated just outside Charlottesville, Virginia (US) in Albemarle County on the summit of Mount Jefferson. It is named for Leander J. McCormick (1819–1900), who provided the funds for the telescope and observatory.

    Arne Henden is a retired American observational astronomer, instrument and software specialist, and co-discoverer of a minor planet. He formerly served as Director of the American Association of Variable Star Observers (AAVSO). The asteroid 33529 Henden is named after him.

    Braeside Observatory is a privately owned astronomical observatory previously owned and operated by Arizona State University. It is located five miles west of Flagstaff, Arizona (US), near the U.S. Naval observatory. The observatory is made up of two buildings, the telescope building and the control building. The telescope building is where the observatory's 16" Cassegrain Reflector telescope is housed. The telescope is controlled from either the control building or from the university through computer controls.

    Anderson Mesa Station is an astronomical observatory established in 1959 as a dark-sky observing site for Lowell Observatory. It is located at Anderson Mesa in Coconino County, Arizona, about 12 miles southeast of Lowell's main campus on Mars Hill in Flagstaff, Arizona.

    <span class="mw-page-title-main">James Melville Gilliss</span>

    James Melville Gilliss was an astronomer, United States naval officer and founder of the United States Naval Observatory.

    The United States Naval Observatory Flagstaff Station (NOFS), is an astronomical observatory near Flagstaff, Arizona, US. It is the national dark-sky observing facility under the United States Naval Observatory (USNO). NOFS and USNO combine as the Celestial Reference Frame manager for the U.S. Secretary of Defense.

    The Astronomical Almanac is an almanac published by the United States Naval Observatory (USNO) and His Majesty's Nautical Almanac Office (HMNAO); it also includes data supplied by many scientists from around the world. It is considered a worldwide resource for fundamental astronomical data, often being the first publication to incorporate new International Astronomical Union resolutions. The almanac largely contains Solar System ephemeris and catalogs of selected stellar and extragalactic objects. The material appears in sections, each section addressing a specific astronomical category. The book also includes references to the material, explanations, and examples. It is available one year in advance of its date.

    Arthur Allen Hoag was an American astronomer most famous for his discovery of Hoag's Object, a type of ring galaxy, in 1950.

    <span class="mw-page-title-main">Kaj Aage Gunnar Strand</span> Danish-American astronomer

    Kaj Aage Gunnar Strand was a Danish astronomer who worked in Denmark and the United States. He was Scientific Director of the U.S. Naval Observatory from 1963 to 1977. He specialized in astrometry, especially work on double stars and stellar distances.

    Víctor Manuel Blanco was a Puerto Rican astronomer who in 1959 discovered Blanco 1, a galactic cluster. Blanco was the second Director of the Cerro Tololo Inter-American Observatory in Chile, which had the largest telescope in the Southern Hemisphere at the time. In 1995, the 4-meter telescope was dedicated in his honor and named the Víctor M. Blanco Telescope; it is also known as the "Blanco 4m."

    <span class="mw-page-title-main">National Astronomical Observatory (Chile)</span> Observatory

    The National Astronomical Observatory of Chile is an astronomical observatory owned and operated by the Department of Astronomy of the University of Chile (UCh). It is located on Cerro Calán, a hill in the commune of Las Condes. The commune is an eastern suburb of Santiago located in Santiago Province of the Santiago Metropolitan Region. OAN was founded in 1852 and became a part of UCh in 1927. The facility on Cerro Calán was completed in 1962.


    1. "National Executive Committee for Space-Based Positioning, Navigation, and Timing". pnt.gov. U.S. Government Printing Office. 2011-06-17. Retrieved 2011-07-27.
    2. "The USNO's Mission". usno.navy.mil. Naval Oceanography Portal. Naval Meteorology and Oceanography Command. Retrieved 2011-07-27.
    3. 1 2 "USNO - Our Command History — Naval Oceanography Portal". www.usno.navy.mil. Retrieved 2022-03-28.
    4. "A Brief History of the Naval Observatory — Naval Oceanography Portal". www.usno.navy.mil. Retrieved 2022-03-28.
    5. Dick, Steven J. (2003). Sky and Ocean Joined: The U.S. Naval Observatory 1830-2000. ISBN   9780521815994 . Retrieved 2013-08-04 via Google Books.
    6. Williams, Frances Leigh (1963). "VIII. Scientific opportunity at last". Matthew Fontaine Maury: Scientist of the Sea. New Brunswick, NJ: Rutgers University Press. p. 164. These different names for the observatory, and the term ‘Hydrographic Office’, were used interchangeably until December 1854, when the Secretary of the Navy officially ruled that the proper designation was “The United States Naval Observatory and Hydrographical office”.
    7. Dick, S.J. (1991). "The origins of the Dominion Observatory, Ottawa". Journal for the History of Astronomy. 22: 45–53. Bibcode:1991JHA....22...31D. doi:10.1177/002182869102200106. S2CID   117369344.
    8. Portolano, M. (2013-03-25). "John Quincy Adams' rhetorical crusade for astronomy". Isis. 91 (3): 480–503. doi:10.1086/384852. PMID   11143785. S2CID   25585014.
    9. Matchette, R.B.; et al. (1995). Guide to Federal Records in the National Archives of the United States. Washington, DC: National Archives and Records Administration.
    10. "Naval Oceanography Portal". www.usno.navy.mil. The James Melville Gilliss Library.
    11. "The Naval Observatory". The Baltimore Sun . 14 December 1842. p. 1. ProQuest   533000734 via ProQuest.
    12. "The new U.S. Naval Observatory, Washington". Monthly Notices of the Royal Astronomical Society . 54 (4): 261. 1894. Bibcode:1894MNRAS..54..261.. doi: 10.1093/mnras/54.4.240 .
    13. "Weekly list of actions, 12/20/2016 through 1/13/2017". National Park Service. Retrieved 2017-01-26.
    14. 1 2 3 4 Dick, Steven J. (2003). Sky and Ocean Joined: The U.S. Naval Observatory, 1830–2000. Cambridge University Press. pp. 547–548, 574. ISBN   978-0-521-81599-4.
    15. Dick, Steven J. (23 May 2005) [2004]. "The American Transit of Venus Expeditions of 1874 and 1882". Proceedings of the International Astronomical Union (abstract). 2004: 100–110. Bibcode:2005tvnv.conf..100D. doi: 10.1017/S1743921305001304 .
    16. "Telescope: Naval Observatory 26 inch refractor". Baltimore, MD: Space Telescope Science Institute.
    17. "The 26-inch "Great Equatorial" Refractor". U.S. Naval Observatory. usno.navy.mil. United States Navy.
    18. "1.0 m Ritchey-Chretien Reflector". nofs.navy.mil. U.S. Naval Observatory Flagstaff. 1998-01-25. Retrieved 2011-07-27.
    19. "History". nofs.navy.mil. USNO Flagstaff Station. Retrieved 2011-07-27.
    20. "1.55 m Astrometric Reflector". nofs.navy.mil. U.S. Naval Observatory Flagstaff. 2001-05-24. Retrieved 2011-07-27.
    21. "Navy Precision Optical Interferometer (NPOI)". Lowell Observatory .
    22. "Paris time by wireless". The New York Times . 22 November 1913. p. 1.
    23. "Catalog information". usno.navy.mil. Naval Oceanography Portal. Naval Meteorology and Oceanography Command. Retrieved 2011-07-27.
    24. "Interactive catalog and image search". usno.navy.mil. Naval Oceanography Portal. Naval Meteorology and Oceanography Command. Retrieved 2011-07-27.
    25. Nemiroff, R.; Bonnell, J., eds. (1999-10-29). "The USNO millennium time ball". Astronomy Picture of the Day . NASA . Retrieved 2015-12-27.
    26. "The James Melville Gilliss Library". usno.navy.mil. Naval Oceanography Portal. Naval Meteorology and Oceanography Command. Retrieved 2011-07-27.
    27. "USNO Master Clock". usno.navy.mil. Naval Oceanography Portal. Retrieved 2011-07-27.
    28. Seidelmann, P.K. (1997). "Nautical Almanac Office 1975–1996". American Astronomical Society Meeting Abstracts. 191: 01.05. Bibcode:1997AAS...191.0105S . Retrieved 2011-07-27.
    29. "The Vice-President's residence and office". archives.gov. U.S. National Archives . Retrieved 2013-02-27.
    30. 1 2 3 4 5 Matsakis, Demetrios; et al. (Civil GPS Service Interface Committee) (2010-09-20). Report from the U.S. Naval Observatory (PDF) (Report). United States Coast Guard . Retrieved 2010-10-31.
    31. "U.S. Naval Observatory declares full operational capability for rubidium fountain clocks" (Press release).
    32. Initial Evaluation of the USNO Rubidium Fountain (PDF). U.S. Naval Observatory (Report). United States Navy. 2006-01-27. Archived from the original (PDF) on 2011-06-15. Retrieved 2010-11-17.
    33. "USNO Network Time Servers". tycho.usno.navy.mil. Retrieved 2011-07-27.
    34. "Telephone Time". usno.navy.mil. Naval Oceanography Portal. Retrieved 2011-07-27.
    35. "The timekeeper behind America's master clock". Washingtonian . 5 December 2012. Retrieved 7 April 2021.
    36. "Keeping time by rubidium at the Naval Observatory". NPR . Retrieved 2015-03-11.
    37. "Telephone Time". www.usno.navy.mil. Naval Oceanography Portal.
    38. "USNO Master Clock via Modem". Tycho.usno.navy.mil. Retrieved 2011-07-27.
    39. Fey, Alan L. "The USNO Instrument Shop". ad.usno.navy.mil. Retrieved 2018-11-08.
    40. Naval Meteorology and Oceanography Command. "U.S. Naval Observatory Special Publications — Naval Oceanography Portal". Usno.navy.mil. Retrieved 2011-07-27.

    Further reading

    Coordinates: 38°55′17″N77°04′01″W / 38.921473°N 77.066946°W / 38.921473; -77.066946