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:
Their longitudes may be reported in three ways:
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.
Pierre (Peter) Charles L'Enfant specified the first meridian in his 1791 "Plan of the city intended for the permanent seat of the government of the United States . . ." (see: L'Enfant Plan). (Shortly after L'Enfant prepared this plan, its subject received the name "City of Washington".) His plan stated near its right side that the longitude of the Congress house, now called the Capitol, was 0,0°.
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 (completed in 1863 during the Civil War) is now given by the National Geodetic Survey as 77°00'32.6"W (NAD 83).
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". (This would place L'Enfant's statue 0.36 meters (1 ft 2 in) north of the latitude of the Capitol.)
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