For many people, wearing a wristwatch has little to do with telling the time. But for those who take that chronometer certification seriously, it’s nice to have a time standard against which to synchronize timepieces and compare rates, if for no other reason than bragging rights. There are countless time signals to use, from the BBC’s hourly pips to the clock on your laptop. But perhaps the most accurate one is the no-nonsense master clock run by the U.S. Naval Observatory. In fact, that time on your computer and iPhone is synchronized to that same master clock.

So what’s behind the USNO’s timekeeping? While we’d love to think it’s a handwound marine chronometer, it is in fact a series of incredibly accurate atomic timekeepers that use oscillations of Cesium atoms to track the seconds, minutes and hours in a day. The time signal is constantly transmitted from the naval observatory to GPS satellites and then bounced back to cell towers on Earth, ending up in our pockets and also at use synchronizing financial transactions, navigation, and military operations. In this short video by The Atlantic, Dr. Demetrios Matsakis, Chief Scientist for USNO’s Time Services, explains how it all works and goes behind the scenes at the facility. Next time you wind and set your humble wristwatch against the time on your MacBook, remember what’s behind those digits.