In a circular plot of land in upper Northwest, the U.S. Naval Observatory has acted as America’s timekeeper for over 150 years. Most people associate the USNO with the Vice President, but Number One Observatory Circle – a house sitting on the grounds, separate from Navy operations – has only been the VP’s official residence since 1974. The real cultural and scientific draw of this institution is its role in using the Sun and stars to create our modern concept of time and navigation.

You’ve probably seen the USNO Master Clock display on Massachusetts Avenue NW, showing the official time of the U.S. Department of Defense (though it’s been blank through the summer). The Naval Observatory is also charged with making and recording precise astronomical and Earth orientation measurements for use in everything from navigational almanacs to GPS satellites. On the grounds is a full observatory, the Master Clock system, an astronomical library, and probably a lot of other strategic military facilities we’re not supposed to walk around in.

The USNO leads tours of the facilities, free and open to the public, just about every Monday night from 8:30 to 10 p.m. If you’re interested, don’t be too impatient, as it can take 4 to 6 weeks to get into an open tour, and they can be canceled with little notice. Nevertheless, a few tours still have space through early December, when the USNO will close for the holidays and reopen on January 10. Be sure you send in the name and date of birth of each guest, as everyone must have a reservation and government ID.