USA Naval Observatory

"Gaze up at the Stars Washington DC is home to the Naval Observatory, the official source of time for the US military and by extension, the country. Computers, cellphones and cable TV boxes all have their clocks set to the time kept here. The observatory offers tours that take place after sunset so you can peer into the large telescope and see the twinkilng stars hanging high up in the sky and you can ask all the questions you want of the astronomers who will be guiding you around. Hands down, this is one of the best FREE tours in town! It is also one of the most difficult tours to sign up for as they fill up weeks in advance. The tours are only offered on select Mondays from 8:30pm to 10pm and they may be cancelled at any time. Reserve as early as you can!"