The Need For Leap Seconds

The above conclusion was not unexpected. We already knew the Moon to be drifting away from us very slowly. The Moon raises tides in the oceans, and these create a drag force that is incessantly dropping the terrestrial rotation rate. Although the effect is small, it is both calculable and observable, for example through radio astronomical observations of distant quasars (these are so far away that they provide unmoving references against which the terrestrial spin may be gauged). On top of this persistent slowdown trend, the rotation rate of the planet is also found to undergo seasonal variations, as the atmosphere swells under summer heating and then shrinks in the winter.

It is because of this general slowing down of the Earth that leap seconds need to be inserted into some years. In the past, time was defined astronomically, from observations of when the Sun and the stars crossed the noon meridian. However, during the twentieth century methods of time determination that were of ever increasing accuracy were developed, eventually resulting in time according to the heavens being abandoned in favor of time according to atomic clocks. The atomic second is the standard we use now, and that is defined according to the length of the day as it was in 1900. Over the century that has elapsed since then, the days have become about 1.7 milliseconds longer. Such differences accumulate to give a discrepancy of one second over 19 or 20 months, making a leap second necessary to keep the time shown by atomic clocks in accord with the spin of the planet. Leap seconds are inserted on an as-needed basis, by international agreement, at the end of either December 31 or June 30.

As the years pass the day is getting longer and longer, and leap seconds will eventually be required more often. If the present rules are maintained then within a few centuries we may need a leap second at the end of every month. One way to avoid this would be to redefine the atomic second in terms of the day length in A.D. 2000 rather than 1900, and then no leap seconds would be needed for some decades, but there are problems with such a solution. For example the fundamental unit of length used in all science and technology, the meter, is now stipulated in terms of how far light travels in a second, and so amending the second would alter the definition of the meter. Also, radio frequencies are given in units of Hertz, or cycles per second, so that changing the second would affect those too.

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