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:
relative to NAD 83, an Earth-centered ellipsoid of revolution with dimensions chosen to best fit the undulating geoid.
NAD83 longitude of the Capitol is about 1.1 arc seconds less than its NAD27 longitude; astronomic longitude there is about 4 arc seconds less than NAD83.
specified the first meridian in his 1791 "Plan of the city intended for the permanent seat of the government of the United States...". His plan stated near its right side that the longitude of the Congress house, now called the Capitol, was. L'Enfant's plan contained the following explanatory note:
In order to execute the above plan, Mr. Ellicott drew a true meridian line by celestial observation, which passes through the area intended for the Congress-House; this line he crossed by another line due east and west and which passes through the same area. These lines were accurately measured, and made the basis on which the whole plan was executed. He ran all the lines by a transit instrument, and determined the acute angles by actual measurement, and left nothing to the uncertainty of the compass.
The longitude of the center of the Capitol's dome is now given by the National Geodetic Survey as.
White House meridian
L'Enfant planned Washington around a right triangle, having its 90° vertex at an equestrian statue of George Washington, its eastern vertex at the "Congress house" and its northern vertex at the "President's house", now named the "White House". The west side of L'Enfant's triangle forms a natural prime meridian passing through the center of the President's house. The following ten features on and near this "Washington Meridian" are listed from south to north:
The center of the Jefferson Memorial, completed on the meridian in 1943.
The Jefferson Pier. In 1793 Secretary of StateThomas Jefferson surveyed and marked with a wooden post the southwest vertex of L'Enfant's triangle, establishing the second Washington meridian, the one through the President's house. The wooden post was replaced by the Jefferson Pier in 1804, while Jefferson was President of the United States. After removal and replacement several times, it was permanently replaced in 1889 by a, 2-foot-tall, granite pier, now WNW of the center of the Washington Monument. NGS gives its longitude as as of 2002. Azimuth to the Capitol is 89.98 degrees, a discrepancy of just under a meter.
The Meridian Stone. Set In 1890 at the center of the Ellipse, it was intended to be on the same meridian. It is an granite post set flush with the ground. NGS gives its longitude as as of 2002.
The Zero Milestone. Set in 1923 on the north side of the Ellipse, it was intended to be on the same meridian and to be the zero mileage point for all United States roads. It is a granite pillar about square and about tall. NGS gives its longitude as as of 2002.
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.
The third meridian was defined on September 28, 1850 by Congress: "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 dome atop the main building of the Old Naval Observatory, now abandoned southwest of the corner of E and 23rd Streets in Foggy Bottom. The observatory adopted for its meridian in the American Ephemeris and Nautical Almanac for the years 1855 to 1869. 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 of the United States.
Many western states have borders that are meridians of "longitude west of Washington", that is, west of the legal 1850 meridian through the Old Naval Observatory. However, their present boundaries follow the subsequently surveyed boundary, even when inaccurately marked a few miles east or west of the meridian in the statute. Other western states have meridians relative to Greenwich or relative to a river or lake. The following meridians west of Washington are listed east to west, along with the appropriate boundary of the present state. The date of the meridian's definition is in parentheses. All states were territories of the same name when their meridians were specified except as noted. States without dates have boundaries dependent on the neighboring state with a date listed immediately above it. 32° west of Washington Meridian, after 2010 reconstruction.
New Naval Observatory meridian
The fourth meridian was through the clock room of the new Naval Observatory, 2.3 miles northwest of the White House, at or or. The clock room is a small building at the exact center of the 1000-foot radius observatory grounds, whose northern entrance is at 34th Street and Massachusetts Avenue. It was used in the Almanac for the years 1898–1950 as the independent variable of time for a few tables.
