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Astro*Dictionary by Michael Erlewine





1 article for "Darwin"

Darwin, Sir George Howard [Astro*Index]

(1845-1912) English astronomer. Born at Down, Kent; died at Cambridge. Second son and fifth child of Charles Darwin and Emma Darwin.

Laplace, with his general elaboration of the Newtonian Gravitational Theory, had explored the matter of the tides in great detail. Darwin took this work a step further, analyzing the irregularities caused by the interference of land barriers and the frictional effects caused by the ocean bottom. Tidal friction would slow the rotation of the Earth and increase its rotation. But, the total angular momentum of the Earth- Moon system must be maintained. Therefore, the angular momentum of the Moon would have to be increased, which requires that the Earth-Moon distance be increased. Thus, the tidal effect would increase the length of the day and cause the Moon to recede from the Earth. This effect would continue, until the length of the day was 55 times its present value. At that point, one part of the Earth would constantly face the Moon, and the lunar tides would remain locked at all terrestrial spots. Extending time into the remote past, the length of the day would be shorter and the Earth-Moon distance would decrease. This effect would continue, until the day was 1/6th of its present value, and the Moon was sufficiently close to the Earth that a collision would occur. By such arguments, Darwin concluded that the Moon had originally been thrown off from the Earth, leaving the hole now known as the Pacific Ocean. This view was supported by analysis of the Earth's outer layers: While much of the Earth's surface displays a granite layer, the Pacific Ocean is free of granite. Astronomers are now faced with at least two alternatives: Was the Moon thrown off from the Earth, or did the Earth and Moon separately evolve? This question is not yet settled.

See also:
♦ Tidal Forces ♦ Earth, Motions of ♦ Moon


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