of the United States. Western state borders 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 (kilometers) east or west of the meridian in the statute. Other western states have meridians relative to Greenwich (Alaska, California, Oklahoma, Texas) or relative to a river or lake (Arkansas, Louisiana, Minnesota, Missouri, Oregon, Washington state). 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 in the 1860s). 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. New Naval Observatory meridian The fourth meridian was through the clock room of the new Naval Observatory, northwest of the White House, at or or . The clock room is a small building at the exact center of the 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 (even though Washington's civil time since 1883 had been that of the standard time zone GMT−5 hours (75°W)). See also Washington mean time International Meridian Conference Notes References Note: Contains much information from book. Van Zandt, Franklin K. Boundaries of the United States and the several states, Geological Survey Professional Paper 909 (1976) American ephemeris and nautical almanac'' (Washington, DC: annual), preface and observatories External links The 1791 L'Enfant plan and the Mall Map of L'Enfant's Plan Washington Monument GPS Project including Jefferson Pier and Zero Milestone (1.29 MB) Washington Refractor describing the condition of Old Naval Observatory in 1972. Washington Monument GPS Height Modernization Project listing coordinates of Jefferson Pier, Meridian Stone, and Zero Milestone (click on names for photos) Meridians (geography) Meridians and base lines of the United States Named meridians Prime meridians