Roland Piquepaille's Technology Trends
How new technologies are modifying our way of life

samedi 20 décembre 2003

We certainly live in a world where everything seems to go faster. But what about the Earth? It's also going faster, at least during the last five years. The National Institute of Standards and Technology (NIST) says it has not inserted an additional leap second since 1999. By contrast, 22 seconds were added between 1972, when we turned to atomic clocks for timekeeping, and 1999.

Before going further, let's look at these atomic clocks, used to keep atomic time synchronized with Earth's time. Below are photos of the first one in 1949 (Credit: NIST) and the current NIST-F1 cesium Fountain clock with its developers Steve Jefferts and Dawn Meekhof (Copyright 1999 Geoffrey Wheeler).

The first atomic clock in 1949 The current atomic clock in 1999
1949 1999

Of course, we can build smaller atomic clocks today. Check for instance a previous post, "ONR Unveils a 'Matchbox' Atomic Clock."

And now, what is a leap second?

A leap second is a second added to Coordinated Universal Time (UTC) to make it agree with astronomical time to within 0.9 second. UTC is an atomic time scale, based on the performance of atomic clocks. Astronomical time is based on the rate of rotation of the earth. Since atomic clocks are more stable than the rate at which the earth rotates, leap seconds are needed to keep the two time scales in agreement.
The first leap second was added on June 30, 1972, and they occur at a rate of slightly less than one per year, on average. Although it is possible to have a negative leap second (a second removed from UTC), so far, all leap seconds have been positive (a second has been added to UTC). Based on what we know about the earth's rotation, it is unlikely that we will ever have a negative leap second.

It may be unlikely, but not a single leap second has been added recently. And NIST scientists don't really know why, even if they have some hypothesis.

Tom O'Brian, a physicist and chief of NIST's Time and Frequency Division in Boulder, Colo., suggests changes in motion of the Earth's core, the effect of ocean tides and weather, and changes in the shape of the Earth may all be affecting the spin of Mother Earth. In general, he notes, the long-term trend has been for the Earth's rotation to slow down, but not in the last five years.
O'Brian said most scientists expect the Earth to continue slowing down again in the future. So maybe there is hope for those feeling particularly harried.

If you want more information on atomic clocks, check this brief history.

Sources: NIST Tech Beat, December 19, 2003; and various NIST pages

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