Sunday, August 5, 2012

Cesium and Maser

By Julie Rahm

The U.S. Naval Observatory (USNO), located in Washington DC, is responsible for determining precise time and managing time dissemination. Modern electronic navigating and communicating systems are increasingly dependent on precise time and time intervals. The Global Positioning System, for example, is based on the travel time of signals from the satellites in space: an accuracy of ten-nanoseconds (ten one-billionths of a second) corresponds to a position accuracy of ten feet! In fast communications, synchronizing time is equally important. Most of these electronic systems are referenced to the USNO Master Clock.  The Master Clock consists of two groups for a total of 44 clocks.  
The first group contains cesium clocks.  They are less accurate than the hydrogen maser clocks of the second group, but more stable in the long term. In short, the radioactive decay of cesium is measured and averaged. To blow your mind, the real definition of a second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom. (Holy Cow!) There are currently of 31 cesium beam clocks operating as a part of the Master Clock.  The average of these 31 cesium beam clocks is used to steer the average of the second group of clocks. The second group contains hydrogen maser clocks. Hydrogen masers are extremely accurate clocks, but only over short time periods (less than one week). 
Automatic comparison of all the clocks every 100 seconds makes the timekeeping accurate and extremely stable. The Master Clock does not change by more than about 100 picoseconds (0.0000000001 seconds) per day from one day to the next. Now the twist! Global Positioning System (GPS) time and Naval Observatory time do not agree! GPS time is ahead by 16 seconds.
The Earth’s rotation is constantly undergoing a deceleration caused by the braking action of the tides. Through the use of ancient observations of eclipses, it is possible to determine the average deceleration of the Earth to be roughly 1.4 hundredths of a second per day per century. This deceleration causes the Earth's rotational time to slow with respect to the atomic clock time.
So, after 500 days the difference between the Earth rotation time and the atomic time would be 1 second. To correct for this difference, a leap second is inserted to bring the two times closer together. Coordinated Universal Time is time corrected for leap seconds. My point is even though the Naval Observatory feeds the time signal into the GPS, GPS time is not corrected for leap seconds. The first leap second was introduced on June 30, 1972. A leap second was inserted into the timekeeping at the end of June. Did you notice? Now you know!
I tell you that, to tell you this: It is your time. What will you do with your seconds today to ensure you are living your best life? Tell me about it at

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