Timekeeping at the U.S. Naval Observatory
The United States Naval Observatory is located at 3450 Massachusetts Ave., NW, Washington, DC.
Beginning in 1934, the Observatory determined time with a photographic zenith tube (PZT), a specialized instrument that points straight upward toward the zenith and automatically photographs selected stars crossing the zenith. This gave a measure of the Greenwich Mean Time (now called Universal Time) , the “time of day” based on the rotation of the Earth. Improvements in clock technology, including the Shortt free-pendulum clock and quartz crystal clocks, soon proved conclusively that the Earth’s rotation was not uniform, and a new uniform time scale known as Ephemeris time came into use in 1956.