United States Naval Observatory, District of Columbia

This historic observatory in the heart of Embassy Row in Washington DC opened in 1893, moving from its original location at Foggy Bottom. It is the official source of precise time for the Navy, which is especially important for calculating longitude for ship navigation, historically. As the observatory at Greenwich, England, is to location, the US Naval Observatory is to time, at least for the USA. The Master Clock is still maintained there, and its signal is used to calibrate satellites and computer systems, though the Air Force has developed its own Master Clock, at Schriever Air Force Base in Colorado, which is used to control GPS satellites, and other assets related to space-based observation and defense. The Navy may have been the keeper of time for the sea, but the Air Force is now the keeper of time for the sky and space. That said, the naval observatory has more clocks than at Schreiver, a total of around 57 cesium atomic clocks, 24 hydrogen masers, and four rubidium atomic fountain clocks. These complex devices operate from more than a dozen environmental chambers, kept at constant temperature and humidity, located at two sites, and connected to various communications systems, including the public internet. Among the dozens of observatory structures at the leafy site, are some that have housed a variety of other functions for the government, including early federal and military mapping agencies. The grounds also have a house that has been the official residence for the Vice President since 1974 (with a bunker, of course). The entire Observatory site is contained within a well guarded circular perimeter, with a radius of 1,000 feet.