This Washington meridian was defined on 28 September 1850 by the United States Congress.
[1] 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).
It was also used to time astronomical events by users of the American Ephemeris and Nautical Almanac, first published for the year 1855.
[3] 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 GMT − 5h8m16.15s) 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.