Leap Seconds

Some of you will be aware that the rotation of the earth is not at a completely uniform speed. In fact, due to the action of the tides and other forces, the earth is slowing down. (Dr. Richard Sauder has written an interesting summary here. ) The international time standard UTC is linked to the earth’s rotation and this means that UTC occasionally has leap seconds added to it to account for the slowing. The last leap second was added just last January.

Why do I care about this? The solar calculation software that I have written depends for it’s high accuracy calculations on what is called dynamical time – time that is determined by an atomic clock, not subject to the vagaries of the earth’s spinning. The Sun API, as the software is called, must convert backwards and forwards between common UTC time and Dynamical Time.

The last time that I updated the conversion tables was in 2002 and I was curious if they were still valid. The International Earth Rotation & Reference System Service has provided this handy summary of the history of the gap between time based on the earth’s rotation and the time kept by atomic clocks.

At the current time there is a 65.187 second gap between UTC and Dynamical Time. It turns out the earth has not slowed at all in the past 5 years (2001-2006) whereas in the previous 5 years, (1996-2000) it had slowed by 2sec. The Sun APIs conversion between UTC and DT was running about 5sec fast for 2006.

If you would like to learn more about the various kinds of time (did you know that there were any?) you can visit: http://tycho.usno.navy.mil/systime.html

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