U.S. Naval Observatory, Time Service Department

“If you want to know where you are you need a good clock.” 

              – Dr. Demetrios Matsakis, Chief Scientist of Time Services

The Time Service building on the grounds of the U.S. Naval Observatory houses the atomic clocks that generate time for accurate GPS navigation.

Located in Washington D.C., The U.S. Naval Observatory was founded in 1830 for the purposes of aiding navigation at sea.  At that time, they calibrated mechanical marine chronometers for ships to successfully locate their longitude.  In short, this required two clocks on the ship; one keeping the ship’s time and one keeping Greenwich Mean Time (GMT).  The ship’s longitude was calculated using the difference between these times. These days, a constellation of GPS satellites uses the time generated at the U.S. Naval Observatory to triangulate global position. 

The accuracy of GPS on your smartphone relies on U.S. Naval Observatory time.  As Dr. Demetrios Matsakis (Chief Scientist of Time Services) explains, “If one satellite was off by one nanosecond compared to the other ones, that would pull your position off by one foot.  Actually, it would be a little bit more if satellites were perfectly aligned.”

Dr. Matsakis stands at Master Clock 1.  Dr. Matsakis explains, "It is THE Master Clock with a capital 'M' for The Naval Observatory. It generates the physical signal that realizes time."  To the right of Master Clock 1 are two Hydrogen Maser Clocks, which are more accurate over a shorter period  of time than cesium clocks.

Master Clock 1

The U.S. Naval Observatory keeps time with over a hundred atomic clocks.  Atomic clocks tell time like almost any other clock – by oscillation (movement back and forth at a regular speed). The swinging of a pendulum in a grandfather clock, the extension and contraction of a hairspring in a mechanical wristwatch and the vibration of the crystal in a quartz watch are all oscillators.  An atomic clock is a clock that gets its time from the oscillations of an atom. In the case of the cesium atom, its radiation oscillates at a very specific frequency: 9,192,631,770 Hz (cycles per second).  These atomic clocks are very accurate; Yet, not the most accurate.

“We also have four clocks here that we built ourselves which are based on rubidium. They're called atomic fountains. These took over ten years and four PhDs to construct.  Any one of them can measure time to about a part in ten to the sixteen, which means that it will take three hundred million years for one of them to lose one second (if they lasted three hundred million years).  I like to say that the four of them together are the most accurate thing ever built by mankind to measure anything day in day out. There's nothing else that we have ever built that measures sixteen-digit accuracy, twenty-four seven.  However, already clocks are being built that are ten to a hundred times better. They just haven't reached the point of going into use yet.  In ten years the clocks that I'm talking about will be quite boring.”


Dr. Matsakis shows us The Naval Observatory's decommissioned Shortt Clock.  Developed by William Hamilton Shortt in collaboration with horologist Frank Hope-Jones in 1921, they were the most accurate pendulum clocks ever produced commercillay.  Shortt Clocks kept time with two pendulums, a master pendulum swinging in a vacuum tank (left) and a slave pendulum in a separate clock (right).  This system allowed the master pendulum to swing with little air resistance and no mechanical disturbance.  It transmitted its time to all the Western Union stations across the U.S. so the country's time be would synchronized.  Around World War II this Shortt Clock was replaced by more accurate quartz crystal clocks.