THE EARTH

Waipa Post, Volume XXIV, Issue 1661, 10 September 1925, Page 2

THE EARTH

ITS LENGTHENING DAY.

The earth is a great clock. Its rotation on its axis affords the means of measuring time. It makes one complete rotation in 23 hours 56 minutes 4.091 seconds. This period is known as the “ sidereal day.” But is the clock reliable? In other words, is the rate at which the clock is spinning constant? Oan the length of the days change? Modern astronomy declares that it can (writes Mr J. A. Lloyd in the Daily Mail).

There is no reason to suppose that the day. has always been what it is. There are good grounds for believing that -it has been very much shorter. The day is slowly but surely lengthening. Mathematicians have shown that at one period of the earth’s career ahe day was only about four hours long.

There are many forces at work that contribute towards slowing down the earth’s rotation. Chief of these is tidal friction. As the waters of the earth flow over its surface, responding to the attraction of the moon (and, to a lesser degree, of the sun), they tend to put a brake on the earth. The braking effect is greater in shallow, narrow seas like the Irish Sea or the Behring Straits than in the deep waters of the ocean. But tides occur in the solid crust of the earth as well as in the seas. The crust is rythmically distorted. The distortion is, of course, exceedingly small, but can be measured by means of the “ interferometer.” It is a startling fact, but quite true, that London itself is lifted up several inches daily and dropped back again. ' The earth’s axis of rotation is also subject to a slight shifting, and this further tends to upset its period of rotation. Then we must consider the effects of eart'h quakes and seismic disturbances generally. These take the form of elevations and depressions of the ocean beds and the shifting of larges masses of land. The combined effect on the earth’s rotation is almost inconceivably small. But imagine the process going on for millions and millions of years! The length of the day has altered even during the time of civilised man. We knoAv this from a study of ancient eclipses, as found in the Chinese records. There is evidence that the length of the day is increasing by about one-hundredth of a second in a century.