The Naval Observatory was created to help mariners navigate the seas. In the 19th century a navigator wanting to know the ship's longitude needed to know the time, which was kept on board ship by a chronometer. Then, says observatory spokesman Jeff Chester, the chronometer itself had to be checked for accuracy.
"And we did that by comparing the rate at which those clocks ran against a clock of known precision, and in the 1800s the best available clock that we had was the Earth itself," he explains.
For that, specialized telescopes called transit instruments such as those at the Naval Observatory were used to observe various stars and note when they passed exactly overhead.
"Then you can use that as the basis to determine how fast or how slow a particular timepiece runs," adds Mr. Chester.
Today, of course, the Naval Observatory uses some of the world's most accurate atomic clocks, but astronomical observations are still done, creating a valuable continuous record of observations.
Online, the U.S. Naval Observatory website has star catalogues and other specialized material for astronomers, but also history about the observatory itself, a feature for stargazers called "the sky this week" and another that might be useful to anyone planning an event.
"One of the most frequently asked questions that I get is, what time is it going to get dark on thus-and-such a day, because I'm planning a wedding or something like that," he recalls. "We have a link where you can put in a date and a location, and it will give you the times of sunrise, sunset, twilight, moonrise and moonset for that particular date and location, anywhere on earth."
Another feature on the Naval Observatory website gives you the height of the sun at any given time or place.
"Architects use this feature when planning energy-efficient houses," he adds. "You want to have an overhanging roof that will shield, say, a large facing-south window or sliding glass door in the summer time, but in the winter time you want to allow sunlight to pass through that, because you want to get some of that solar heat in the house in the dead of winter."
And of course, the website of America's timekeeper also provides the correct time of day at usno.navy.mil.