EARTH'S LENGTHENING DAYS

Hokitika Guardian, 7 February 1927, Page 4

EARTH'S LENGTHENING DAYS

(By J. A. .LLOYD, .M.8.A.A., in the London "Daily Mail”.) The earth is a great clerk. Its rotation on its axis affords the means oi' measuring time, it makes one complete rotation in 23 hours 50 mttiutes • 1.0!)I seconds. This period is known as the " sidereal day.” But is the clock reliable? In other words, is the rate at which the earth )s spinning constant? Can the length of the days change? Modern astronomy declares that it can.

There is no reason to suppose that the day has always been what it is. There are good grounds for believing it has been very much shorter. The day is slowly but surely lengthening. ...athematieians hav shown that at one period of the earth’s career the day was only about four hours long. There are many forces at work which 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 t/> put a brake on the earth. The braking effect is greater in .shallow. narrow seas like the Irish Sea or the liehring 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 rhythmically 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 earthquakes and seismic disturbances generally. These take the form of elevations and depressions of the ocean beds and the shifting of large 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. Wo know 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-hundred of a second in a < e"turv.