Old USNO with Time Ball
The time ball was one of the first systems to enable the Observatory to disseminate time to support remote users. The ball was dropped daily (except Sundays) at the astronomically determined instant of Mean Solar Noon in Washington, which enabled the navigators of ships anchored in the Potomac River to check the rates of their chronometers.  Depicted above is the time ball atop the dome of the 9.6-inch refractor telescope at the USNO's Foggy Bottom site, its home from 1844 until 1893.
 
The U.S. Naval Observatory is one of the oldest scientific agencies in the country. It was established in 1830 as the Depot of Charts and Instruments. Its primary mission was to care for the U.S. Navy's chronometers, charts and other navigational equipment.
 
In 1844, as its mission evolved and expanded, the Depot was reestablished as the U.S. Naval Observatory and was located on a hill north of where the Lincoln Memorial now stands in Washington's Foggy Bottom district. For nearly 50 years the Observatory remained at the Foggy Bottom location. During these years significant scientific studies were carried out such as speed of light measurements, the phenomena of solar eclipses, and transit of Venus expeditions. Publication of its annual astronomical and nautical almanacs started in 1855. In 1877, while working for the Naval Observatory, astronomer Asaph Hall discovered Phobos and Deimos, the two satellites of Mars.
 
USNO Building 1
 The U.S. Naval Observatory's historic Building 1, designed by Richard Morris Hunt
and completed in 1893

 
However, by the late 1870s, it was clear that the Naval Observatory had to move out of the city. Unhealthy conditions in the Foggy Bottom neighborhood had taken their toll, with annual bouts of malaria originating from the swampy banks of the Potomac River in the warmer months of the year. In 1893, after nearly 50 years at the site on the river, the Observatory moved to its present location in the hilly terrain north of Georgetown. At that time, this rural site was well outside the city in the farm land above Georgetown. The move not only provided better astronomical observing conditions, but also provided an opportunity to rethink old scientific programs and propose new ones. Along with the new programs such as daily monitoring of solar activity, the old functions of time keeping and telescopic observations were kept intact when the Observatory moved to the new site. The old Observatory in Foggy Bottom was declared a National Historic landmark in 1966 and is currently a part of the U.S. Department of State.
 
As remote as USNO's the new site was, however, it soon became surrounded by the expansion of the city of Washington.  By the late 1940s the operation of the Observatory's newest and largest telescope, the 1-meter Ritchey-Chretien reflector, was severely hampered by the increasing light pollution of the encroaching urban development.  By 1955 a new site for the telescope was selected a few miles west of Flagstaff, Arizona, and the Naval Observatory Flagstaff Station (NOFS) was established.  NOFS now hosts the USNO's largest telescopes and conducts pioneering research into the positions of nearby low-mass stars and solar system bodies, observing in the optical and infrared regions of the electromagnetic spectrum.
 
USNO's time determination and distribution functions have also evolved over the years.  From dropping a "time ball" at the Foggy Bottom site to maintaining the world's most advanced suite of atomic clocks, the Naval Observatory's Master Clock time-scale is distributed to the world through the DoD's Global Positioning System.  To maintain the robustness and security of this vital asset, USNO operates and maintains an Alternate Master Clock (AMC) at Schriever Air Force Base in Colorado.
 
Today, the U.S. Naval Observatory is the preeminent authority in the areas of timekeeping and celestial positional observing, determining and distributing the timing and astronomical data required for accurate navigation and fundamental astronomy.
 
A more extensive history of the Observatory may be found here.
 
The definitive history of the U.S. Naval Observatory is the book Sky and Ocean Joined: The U.S. Naval Observatory 1830 - 2000 by Steven J. Dick (Cambridge, UK, Cambridge University Press, 2002, ISBN 0 521 81599)
 
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