Abstract
IN a supplement to this issue we publish an article by Prof, de Sitter in which a comparison is made of the time as determined by the rotation of the earth, the revolution of the moon round the earth, and of the inner planets round the sun. The application of the theory of gravitation and the laws of motion to the various bodies shows that equal intervals of time determined by one body are not exactly equal according to the others. The application of the laws of dynamics is complicated by the fact that the earth is not a rigid body and that tidal friction is slowly decreasing the rate of the earth's rotation and at the same time that of the moon's revolution. It is not possible to compute the numerical coefficient exactly, but work by Jeffreys on the tidal friction in narrow seas gives a coefficient of the right order of magnitude. Prof, de Sitter finds that the observations are best explained if the coefficient for the three intervals, before 1745, 1745-1870, after 1870, are approximately in the ratio 2:1:3. It is, however, difficult to explain why the coefficient should have altered to this extent. Tidal friction can only slow down the earth's rotation, while observations of the moon and planets indicate that at times this rotation is accelerated. This can only be done by reduction of the earth's moment of inertia. Prof, de Sitter finds that the reduction of the whole mountain range of Central Asia to sea-level would have produced only one-quarter of the change in the length of the day which observations of the moon indicate took place in the year 1918, but that an alteration of the earth's radius by five inches would suffice. Both explanations almost appear to call for observable effects on the earth itself.
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News and Views. Nature 121, 108–113 (1928). https://doi.org/10.1038/121108a0
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DOI: https://doi.org/10.1038/121108a0