Washington Mean Time was the time at the meridian through the center of the old dome atop the main building at the old

US Naval Observatory United 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. Established in 1830 as the Depo ...

Washington, D.C. ) , image_skyline = , image_caption = Clockwise from top left: the Washington Monument and Lincoln Memorial on the National Mall, United States Capitol, Logan Circle, Jefferson Memorial, White House, Adams Morgan, ...

This

Washington meridian The Washington meridians are four meridians that were used as prime meridians in the United States and pass through Washington, D.C. The four which have been specified are: # through the Capitol # through the White House # through the old Naval Ob ...

was defined on 28 September 1850 by the

United States Congress The United States Congress is the legislature of the federal government of the United States. It is bicameral, composed of a lower body, the House of Representatives, and an upper body, the Senate. It meets in the U.S. Capitol in Washing ...

. The Old Naval Observatory is now on the grounds of the United States Navy Bureau of Medicine and Surgery, southwest of the corner of E and 23rd Streets in

Foggy Bottom Foggy Bottom is one of the oldest late 18th- and 19th-century neighborhoods in Washington, D.C., located west of the White House and downtown Washington, in the Northwest quadrant. It is bounded roughly by 17th Street NW to the east, Rock Cr ...

(north of the

Lincoln Memorial The Lincoln Memorial is a U.S. national memorial built to honor the 16th president of the United States, Abraham Lincoln. It is on the western end of the National Mall in Washington, D.C., across from the Washington Monument, and is in the ...

). Washington Mean Time was sometimes called Washington Meridian Time. It was never used as the basis of any time zone, although it was the

local mean time Local mean time (LMT) is a form of solar time that corrects the variations of Solar time#Apparent solar time, local apparent time, forming a uniform time scale at a specific longitude. This measurement of time was used for everyday use during the ...

of the city of Washington before the advent of American time zones on 18 November 1883. It was also used to time astronomical events by users of the ''

American Ephemeris and Nautical Almanac ''The American Ephemeris and Nautical Almanac'' was published for the years 1855 to 1980, containing information necessary for astronomers, surveyors, and navigators. It was based on the original British publication, '' The Nautical Almanac and Ast ...

'', first published for the year 1855. In 1897, well after the Old Naval Observatory closed in 1892, the Coast and Geodetic Survey reported that its meridian was 77°3′2.3″ west of Greenwich, which was quoted for the next 50 years in the list of observatories in the Almanac as . This old Washington meridian was repealed on 22 August 1912.37 Stat. 342 A later version of Washington mean time based on the meridian of the clock room at the exact center of the New Naval Observatory (77°4′2.24″W or ) was still being used in 1950 on a few pages of the ''American Ephemeris and Nautical Almanac'', even though most of its pages used Greenwich Civil Time, the American name for the midnight epoch Greenwich Mean Time. For astronomical purposes, before 1925 a day was considered to start at noon rather than the previous midnight. Thus to convert times of astronomical events before 1925 given in Washington mean time to modern

Universal Time Universal Time (UT or UT1) is a time standard based on Earth's rotation. While originally it was mean solar time at 0° longitude, precise measurements of the Sun are difficult. Therefore, UT1 is computed from a measure of the Earth's angle with ...

it is necessary to add an additional 12 hours beyond the meridian difference from Washington to Greenwich, totalling more than 17 hours.

References

{{reflist

External links

Position of the Observatory
in ''Observations made during the year 1889 at the United States Naval Observatory'' (Washington: 1893) page XXII

describing the present condition of the Old Naval Observatory

* ttps://web.archive.org/web/20060819114851/http://www.state.nd.us/ndgs/Boundaries/Boundaries.htm North Dakota's BoundariesIts western border is 27° west of the Washington meridian
State lines and how they were surveyed
Time zones Time in the United States