Abstract
INTEREST in the dynamical history of the Earth–Moon system was stimulated by the calculations of Gerstenkorn1, who deduced that the Moon was originally an independent planet in a solar orbit close to that of the Earth and that it was captured into a highly elliptical retrograde terrestrial orbit of small perigee. The orbit was brought closer and its ellipticity was reduced by tidal interaction until a dramatically close encounter with the Earth flipped the Moon over into a prograde orbit, from which it has receded to its present position under the influence of tidal friction. Gerstenkorn's theory has been favourably reviewed by Alfven2,3 and very similar dynamical histories are deduced in several more recent calculations4–6. An exact estimate of the time since the very close approach of the Moon is not possible because tidal friction is dependent on several factors, including variable geometry of the oceans. Estimates vary between 2.5 × 109 yr and 1.4 × 109 yr (refs. 1 and 4).
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