Adjusting Our Atomic Clock for the Leap Second

Posted on Thu, May 12, 2016
  • by: Andrew Johnston, Research Associate

Our exhibition Time and Navigation features an atomic clock that will keep an accurate time within a tiny fraction of a second for the foreseeable future (see my earlier post to learn how atomic clocks work and how we installed ours into the exhibition). Except, of course, when we need to account for a leap second.

What’s a Leap Second?
In the past, time was measured using the rotation of the Earth. With atomic clocks, we learned that the length of a day changes by a second here and there. To take this into account, in 1972 the International Telecommunications Union adopted “leap seconds.” A leap second is added whenever the Earth’s rotation gets out of sync compared to the international time reference measured with atomic clocks. There is a discussion about leap seconds and if they will continue to be added, but that is another story. For our purposes, we just needed to keep our atomic clock in sync with the world’s time when the last leap second was added in June 2015.

Adding the Leap Second
Because our atomic clock is not connected to outside data sources, we had to add the leap second manually. This required opening the front of the exhibit case and typing a series of commands on the keypad a few days before the leap second would occur. We told both the frequency standard and the time code generator to add the leap second on the last day of June. We also successfully tested the cable connection in the back of the atomic clock. From now on, we’ll use that to update the clock for future leap seconds.

Leap seconds are always added at midnight Coordinated Universal Time (UTC). For us, that was 8:00 pm in Washington, DC. With our commands entered earlier, I waited around in the Time and Navigation gallery to see what would happen as the clock struck 8:00. The leap second went off without a hitch.

It was interesting to watch the displays.

UTC on the cesium clock displayed these seconds:


Above the clock, local time from the time code generator displayed:


This video shows the time displays 5 seconds before and after 8:00 pm. The elapsed time was 11 seconds instead of 10 because of the leap second.

Our exhibition Time and Navigation includes an atomic clock. On June 20, 2015, the clock needed to be adjusted to account for a global leap second. 

Please come visit the Time and Navigation exhibition to see the working atomic clock. You can set your watch to the correct local time displayed in the upper part of the case.

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