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 at Washington, D.C. This Washington meridian was defined on 28 September 1850 by the United States Congress (9 Stat. 515). 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 (north of the Lincoln Memorial). 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 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, first published for the year 1855.
The difference between Washington mean time and Greenwich Mean Time was determined to be 5 hours 8 minutes 12.04 seconds in 1884 by the U.S. Coast and Geodetic Survey. This is equivalent to 77°3'0.6"W. However, most sources cite 77°3'2.3"W as the Washington merdian to which the borders of several western states are referred. The difference may be due to polar motion which was not yet recognized in 1884. This old Washington meridian was repealed on 22 August 1912 (37 Stat. 342). Nevertheless, Washington mean time 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 it is necessary to add not 05:08:12.04 but 17:08:12.04.
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