Our Astronomical Column

Abstract

THE ACCELERATIONS OF THE SUN AND MOON.—The Journal of the British Astronomical Association for January contains an address by Dr. Harold Jeffreys on this subject. He starts by quoting the results obtained by Dr. J. K. Fotheringham from ancient observations of eclipses and other phenomena (Mon. Not. R.A.S., December, 1920), viz. 21.6″ for the moon and 3″ for the sun. These are the velocities gained per century per century; on the less logical system that gives the space gained in a century the figures are halved. Of the lunar figure 12.2″ is due to the diminution of eccentricity of the earth's orbit. The remaining 9.4″ for the moon and 3″ for the sun are ascribed to tidal friction, which diminishes the earth's rotational speed, thus lengthening the day. It would, at first sight, appear that the effect on the moon should be thirteen times that on the sun, this being the ratio of their mean motions. Since, however, the mutual action of moon and earth does not alter the moment of momentum of the system, a retardation of the earth's rotation is accompanied by a recession of the moon and the consequent lengthening of her period, which cancels a considerable part of the apparent acceleration due to the slower rotation. Dr. Jeffreys notes that the theoretical values of solar and lunar accelerations due to tidal friction are uncertain, and may be anywhere between i to 3 and i to 10. He then describes in detail the recent work of Major G. I. Taylor and himself (already described in this column) which determined the regions on the earth's surface where the friction is taking place; the Bering Sea is the largest contributor, but the action in the Irish Sea is quite sensible.