was). It is a granite pillar about square and about tall. NGS gives its longitude as (likely error less than two centimeters). The center of the White House. Clark Mills' equestrian statue of President Andrew Jackson in Lafayette Park, erected on the meridian in 1853 (see: Andrew Jackson (Mills)). 16th Street Northwest, which extends due north from the White House. The meridian is sometimes identified as the "16th Street Meridian" because of the location of this street. Meridian Hill. In 1804, a small freestone obelisk was placed at the crest of a hill, north of the President's House. The marker, which no longer exists, was at the northern end of 16th Street, just north of Florida Avenue, before 16th Street was extended northward about 1890, covering it up. The park along the east side of 16th Street where the obelisk once stood still bears the name Meridian Hill Park. District of Columbia entrance marker stone (approximately east of this meridian) near Silver Spring in traffic circle (Blair Circle) at intersection of 16th Street Northwest, Eastern Avenue Northwest, N. Portal Drive Northwest and Colesville Road. Old Naval Observatory meridian The third meridian was defined on September 28, 1850, by Congress: "[T]he meridian of the observatory at Washington shall be adopted and used as the American meridian for all astronomical purposes and ... the meridian of Greenwich shall be adopted for all nautical purposes." The observatory decided that this meridian passed through the center of the original (small) dome atop the main building of the Old Naval Observatory, now abandoned southwest of the corner of E and 23rd Streets in Foggy Bottom (north of the Lincoln Memorial and west of the White House). The observatory adopted for its meridian in the American Ephemeris and Nautical Almanac for the years 1855 to 1869 (as . In 1897, well after the observatory closed in 1892, the Coast and Geodetic Survey reported that its meridian was west of Greenwich, which was quoted for the next 50 years in the list of observatories in the Almanac (as . When referred to later datums, this meridian has been variously specified as or (both presumably NAD 27). If NAD27, the latter would be which seems to be within a few meters of the actual longitude, at worst. This meridian was repealed by Congress on August 22, 1912, to allow the Greenwich meridian to become the legal prime meridian