The Transits of Venus by William Sheehan and John Westfall Prometheus Books, $28, ISBN 1591021758
THERE will be a transit of Venus on 8 June. The planet will pass directly between the sun and the Earth, and for a few hours will appear as a small black disk crawling very slowly across the brilliant face of the sun. It will certainly not be spectacular, but it will be watched with great interest.
Its fascination lies in its infrequency: the last transits occurred in 1874 and 1882. They occur in pairs separated by eight years, then no more appear for over a century, so there can be no person now living who can remember seeing one.
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William Sheehan and John Westfall, two distinguished American astronomers, have produced a timely book dealing with all aspects of the phenomenon. Venus, unlike Mercury, can be seen with the naked eye while in transit, yet the first scientific observations date back only to 1639, when the transit was recorded by only two astronomers, Jeremiah Horrocks and William Crabtree. All later transits were carefully observed, because they provided what was then the best method of measuring the length of the astronomical unit: the distance between the Earth and the sun. All these transits are described in detail, and there is also a great deal of interesting miscellaneous background information, though some of it is only marginally relevant to the main theme.
Transits of Venus have played a role in world history. The main mission of Captain Cook鈥檚 first epic voyage was to take astronomical observers to Tahiti so that they could make exact measurements of the 1769 transit. It was only after this that he went on to Australia.
Less fortunate was the French astronomer Legentil, who journeyed to the Far East to observe the transit of 1761. The expedition was delayed by wartime activities, and he was forced to watch the transit from the deck of a ship, failing to make any scientific measurements. Not to be daunted, Legentil stayed abroad for eight years so that he could observe the 1769 transit instead. Alas, he was clouded out, and finally returned to France to find that he had been presumed dead, and his heirs were preparing to distribute his property. Anecdotes of this sort make the book a good read, and the excellent list of references means that it will also be of value to the serious student.
It is fair to say that the transit method was never really successful in measuring the length of the astronomical unit; there were too many observational uncertainties. Now, of course, the method is obsolete, and the next transits are not regarded as scientifically important. Yet many will still want to watch, and if you do, you would be wise to read this excellent book before the events this summer.
The last chapter, 鈥淎 compleat guide to the transits of 2004 and 2012鈥�, details what to look for and how to make observations. The authors are careful to stress the risks to eyesight posed by solar observation, and describe how to make use of photographic and photoelectric equipment. Weather permitting, the whole of the 2004 transit should be visible from Europe, Africa and Asia, while the transit of 2012 will be best observed from the southern hemisphere. After that, we must wait until 2117 for another chance.