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The "leap second" lives!

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I am disappointed to hear that the ITU has not killed one of the stupidest ideas in timekeeping at its meeting in London today. Instead the decision has been postponed to the 2015 meeting.

As you might have noticed, the earth is not a clock. While it takes the earth almost exactly 24 hours to revolve so that it is facing the sun in the same way the movement is not perfectly regular. Changes in the shape of the earth cause it to speed up or slow down. The tidal motion of the moon acts as a brake and slows it down very slightly. Lesson: do not attempt to use a very large, very heavy physical object as the basis for a precise system of measurement.

The discrepancy is less than a second per year but some people worry that over time these seconds might add up and cause some sort of catastrophe and so for the past fifty years the ITU has synchronized the UTC time count to the rotation of the earth by adding (and occasionally subtracting) leap seconds.

One consequence of this practice is that is not possible for computer systems to precisely calculate the number of seconds to any future date and time that is more than a year into the future. There might be 31,536,000 seconds between 00:00:00 on 21st January 2013 and 00:00:00 on 21st January 2014 or there might be 31536001. That type of unknown can cause major problems in planning systems.

Dealing with the leap second requires complex code and the ability to update the database of leap seconds. When two computer systems interact there is always a possibility of error being introduced because one is applying a correction and the other does not.

Astronomers have known about this problem for decades and that is why the astronomers I worked with at Oxford used International Atomic Time (TAI) rather than the error prone UTC. Using UTC in a scientific context is like using feet and inches to measure distances: Serious scientists just don't do it when it would make a difference.

If leap seconds are abolished it will take approximately 7200 years before the discrepancy between UTC and 'Universal Time' (time corrected to the rotation of the earth) reaches an hour. Why should this matter to anyone? According to current custom we move the clocks forwards and backwards by an hour twice a year. New York is not on the precise same longitude as Boston. In the early Victorian era, the two cities would have used different time as a matter of course. Today we ignore the difference because the social cost of maintaining separate local time would be huge, the value nil.

Attempting to determine longitude by using the equation of time has always required the use of correction factors. Depending on latitude the time of solar noon varies by several minutes over the course of a year in any case. The orbit of the earth is not circular and the rotation of the earth is tilted with respect to its orbit.

The leap second is an archaic notion that has absolutely no practical value and causes real practical difficulties. The ITU should have had the guts to kill it off, instead they procrastinated.

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