Decoupling civil timekeeping from Earth rotation


I spend quite a lot of time thinking about leap seconds. They’re the extra seconds inserted into time at midnight on New Year’s Eve every few years.

Recently they’ve been hitting the news, as a proposal led by some American time scientists to abolish the leap second system has caused a bit of a furore.

OK, here’s the deal. For ever, we’ve measured time by the rotation of the Earth. One rotation equals one day. At least, that was the case until Louis Essen and Jack Parry developed the first practical atomic clock, in 1955 (you can see it at the Science Museum).

The first caesium atomic clock, 1955 (© Science Museum/SSPL)


This technology, once developed and refined, became a more accurate timekeeper than the spinning Earth, so we moved to atomic clocks to measure time.

But we’re animals, and we’re therefore hard-wired to the temporal patterns of the Earth’s rotation – daylight and darkness, the seasons. So a system was designed to keep the new atomic timescale in step with the Earth.

That’s the leap second, and we’ve been recalibrating the atomic clocks with these occasional one-second corrections since 1972. The resulting timescale is called Coordinated Universal Time, or UTC, and it’s never more than a fraction of a second away from Greenwich Mean Time.

So the proposal by the American scientists to abolish the leap second system and let our civil timescale drift away from GMT has set a lot of people thinking about fundamental issues in the philosophy, technology and metrology of timekeeping.

My point? Recently I read about a colloquium held last October in Pennsylvania exploring the implications of redefining UTC as a purely atomic timescale – decoupling civil timekeeping from Earth rotation.

You can read the papers and discussions here (click Preprints). This is truly fascinating stuff – measured, thoughtful and far-reaching, and the first substantial engagement I have read on the implications of this challenging proposal. Well worth a read if you’ve a spare second.


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One Response to Decoupling civil timekeeping from Earth rotation

  1. Ray Essen says:

    Leap seconds are connected with the story of the transfer from astronomical to atomic time so I thought it may be interesting to reflect a little on their origin.

    The time of day is not required with any great accuracy for most civil purposes. Time intervals, on the other hand, are required to be as precise and uniform as possible, particularly for air navigation, computing, secure communications and many other location-based technologies. These two requirements are so different that it might be asked why two sets of time signals are not used, giving astronomical time for civil purposes and atomic time for everything else. The fundamental objection to this is that it would constitute a duplication of one of the fundamental units of measurement.

    The first step to coordinate astronomical and atomic time was taken on 1st January 1958 when the two timescales were made to coincide. This decision was based on the joint work of astronomer William Markowitz (US Naval Observatory) and physicist Louis Essen (National Physical Laboratory). It was immediately realised that the timescales would diverge because of variations in the rate of rotation of the earth and the question to be resolved was the amount of divergence that could be tolerated.

    Several provisional solutions were tried before international agreement was reached to re-define the SI unit of time (the ‘second’) in terms of a property of the caesium atom (1967) and to permit the addition or subtraction of a ‘leap’ second into UTC (1972).

    By the way, the longest year in the 20th century was 1972; it was a leap year and also had two leap seconds inserted at the end of 30 June and 31 December.

    David Rooney mentioned the first practical atomic clock at the Science Museum. If you would like to know more about this clock, I have unearthed a rare film from the 1950s which can be viewed on the NPL YouTube channel (

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