DID you adjust your watch on New Year's Eve? Strictly speaking you should have put it back by one second, just before the final "pip" to announce the birth of 1996. This extra "leap second" was decreed to ensure that our clocks do not deviate unacceptably from the average "day" as defined by the rotational period of the Earth.
Contrary to what we conveniently assume for everyday activities, the time taken for the earth to rotate on its axis is not exactly 24 hours. Indeed looked at on a time-scale of hundreds of millions of years, the day has been getting shorter by about one second's every 60,000 years, as over the millenia the earth spins progressively more slowly on its axis.
We blame the moon for this interference with our daily span. Its gravitational pull produces tides both in the oceans and in the earth's solid crust: as the planet rotates, water is dragged by the tides across surface of the earth, and layers of rock rub against each other as they rise and fall.
The net result is a very slow conversion of the earth's rotational energy into frictional heat. Six hundred million years ago, for example, the day was only 20 hours in length, and there were 428 of them in every year.
But within this progressive trend for shorter days, there are irregular fluctuations as the earth, for various reasons, increases or slows down its rate of spin. Careful measurements have shown, for example, that in the last 100 yeas or so the average day was at its shortest in the l860s, when it was eight-thousands of a second less than in the early l900s: from 1910 until about 1930 the earth accelerated, shortening the average day, and then slowed down again to provide the longest average days in recent times during the early l970s.
These, however, are average figures: during any individual year the length of the solar day varies seasonally by a much greater amount. This occurs partly because the earth moves slightly faster in its orbit at certain times of year than it does at others, and partly - believe it or not - because of variations in the weather.
The winds of the world are at their most vigorous in the period corresponding to our northern winter: this gives the atmosphere a greater angular momentum around the earth's axis, which must be compensated for by a decrease in the rate of rotation of the earth itself.
The end result is that the "24-hour" day is a full 30 seconds longer at this time of year than it is in summertime.