U.S. Naval Observatory

Please visit The U. S. Naval Observatory for information on the following ... 

The year 1877 promised to be an exciting timefor astronomers interested in the study of Mars. The Red Planet was to be in perihelic opposition, the closest position that exists between Mars and Earth. In the two decades since the last perihelic opposition, a new 26-in (66-cm) refracting telescope had been built at the U.S. Naval Observatory in Washington, D.C. The man in charge of the telescope, Asaph Hall, had decided to focus his efforts on finding out whether Mars had any satellites. 

Even the tiniest inaccuracies in the measurement of the time interval between when the signal is sent and when it is received can cause substantial errors— differences of several feet or more— in the triangulation calculation that determines the user s physical location. As a result, GPS receivers rely heavily on standard atomic frequency references. Each satellite has a small handful of local atomic clocks onboard (multiple clocks are used to provide a system of redundancies, a way to ensure reliability and accuracy). Each clock is calibrated to a master atomic clock measuring UTG and managed by the U.S. Naval Observatory. In general, each clock on a GPS receiver, no matter where on Earth it is located, will be no more than 500 nanoseconds to 1 millisecond out of sync with UTG. Time measurement not only facilitates the movement of cars, boats, and airplanes on the earth... 

The Astronomical Ephemeris (formerly The American Ephemeris and Nautical Almanac), published annually by the U.S. Naval Observatory and the U.K. Alamanac Office contains the most current available data on orbital dynamics of solar system bodies. 

F. J. VRBA and H. D. ABLES U.S. Naval Observatory Flagstaff, Arizona... 

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