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Is it true that the first scientific determination verifying the longitude of Washington DC was made at Georgetown University?

Yes. In 1846, James Curley, S.J., calculated the longitude and latitude of the newly constructed Observatory on campus and, by triangulation, that of significant sites in Washington, including the White House and the Capitol. His calculations, made in collaboration with astronomers at the Royal Observatory in Greenwich, England and at the Naval Observatory in Washington, were based on observations of moon transits. They became accepted for the city of Washington, D.C. Decades later, after the laying of the transatlantic telegraph cable, the longitude of Washington was redetermined using telegraphic time signals. This redetermination showed Father Curley’s observation of longitude to be accurate to within three tenths of a second.

Fr. Curley began teaching natural philosophy and mathematics at Georgetown in 1831. In 1841, he proposed the construction of an observatory on campus, drew up plans for it, and supervised the construction work. He taught at Georgetown until 1879 and died here on July 24, 1889. The Georgetown University Observatory—one of the first fixed astronomical observatories in the U.S.—was used by astronomers to study and map the skies until the closing of the University’s Astronomy Department in 1971.