The Big Splat: How our Moon came to be by Dana Mackenzie, John Wiley, 拢17.50/$24.95, ISBN 0471150576
IF YOU stayed up late to watch the total lunar eclipse on 16 May you may have found yourself staring at the Moon and wondering how it got there. If so, Dana Mackenzie鈥檚 The Big Splat offers an array of intriguing theories on just that.
The problem of the Moon鈥檚 origin has perplexed astronomers for a great many years. All of the numerous theories proposed have been found to suffer from serious flaws. The latest, largely attributed to William Hartmann, Donald Davis, Alastair Cameron and William Ward, assumes that the Moon was created following a collision between the early Earth and a wandering planetary body perhaps the size of Mars. It says that the debris from the collision went into orbit around the Earth and then coalesced into the Moon. This is the giant impact theory 鈥� Mackenzie鈥檚 鈥淏ig Splat鈥�.
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Mackenzie runs through astronomical history in the first three chapters, but not all of it is strictly relevant to the main theme. There are some minor slips: for example, the observatory built by Tycho Brahe, last of the great pre-telescopic observers, was not in Norway but on Ven, an island between Denmark and Sweden. And Edmond Halley did not discover Halley鈥檚 Comet, but instead was the first to work out its orbit.
Interesting comments abound about astronomers, past and present, though whether all of these are strictly authentic is debatable. Thus Harold Urey, the Nobel prizewinning chemist, is described by an unnamed contemporary as having had 鈥渁 bulldog face and a bulldog mind鈥�, and as 鈥渇orever breaking off relations with other people for slights, real and imagined鈥�. This is not my recollection of him, and I knew him fairly well.
The Big Splat describes other, older theories in some detail. George Darwin, son of Charles, believed that the Moon was simply thrown off from a rapidly rotating Earth. It was subsequently claimed that this accounted for the deep basin now filled by the Pacific Ocean. When the idea was shown to be untenable, it was proposed that the Earth and the Moon were born together, but independently, at the same time and from the same material surrounding the youthful sun. Or could it be that the Moon was once a completely independent body, and was captured by the Earth? All of these theories were examined, and found to have very serious weaknesses.
Then came the popular giant impact theory. There is a great deal to be said in its favour, and there do not seem to be any fatal objections to it.
Mackenzie is convinced that the giant impact picture is correct, referring to the wandering body as Theia. Certainly his arguments are convincing, and very well presented, but he does give the impression that the question has been definitely settled, and this is not so. While the theory is supported by many 鈥� perhaps most 鈥� authorities, many details are missing. So in some respects, the jury is still out.
There are other minor criticisms: surely it is not necessary to waste several pages disposing of the absurd theory that the Moon landings never happened? All in all, though, this is a most useful and interesting book, far better than its title would suggest.