The Prime Meridian runs through the Royal Observatory, Greenwich, England. Half of a great circle that divides Earth into the Eastern and Western Hemispheres, it is the line of longitude defined to be 0°.
Astride the Prime Meridian, Royal Observatory, Greenwich.
Unlike the equator and the corresponding parallels of latitude, which are set by Earth's rotational axis, the Prime Meridian has no natural analog. Its location is essentially arbitrary, and throughout history different nations have established or advocated various meridians to serve as standard reference lines.
The United States did not adopt the Greenwich Meridian as its standard for all purposes until 1912. From 1850 until then, the nation's official meridian—the American Meridian—was the line that passed through the center of the original dome atop the main building now known as the Old Naval Observatory. Located on the southwest corner of E Street and 23rd Street, N.W., in Washington, D.C., the observatory buildings are presently home to the U.S. Navy Bureau of Medicine and Surgery, and the grounds are closed to the public.
Barely visible through the trees, the dome of the Old Naval Observatory once defined the location of the American Meridian.
Congress specified that ". . . the 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."
About four blocks north of the former observatory, on the campus of George Washington University, a stone line set in the sidewalk at H Street and 24th Street, N.W., commemorates the location of the American Meridian.
The view southward of the stone line commemorating the American Meridian on the George Washington University campus.
A nearby plaque notes that this meridian served as the reference used to survey the boundaries of several western states, including Wyoming's eastern and western borders (29° west and 36° west), Colorado's eastern and western borders (27° west and 34° west), and Oregon's eastern border (42° west).
Photos by I. Peterson