June 8, 2004: Venus in Transit

Field: Physics and Astronomy (Physical Sciences)

A transit of Venus is a relatively rare astronomical event in which the silhouette of Venus is seen to move across the face of the Sun. The phenomenon typically lasts several hours, during which Venus is seen as a small dot against the half-degree angular diameter of the solar disc. The last transit of Venus occurred in 1882; the next will be 8 June 2004. Such transits were once of great importance in astronomy. By observing a transit simultaneously from well separated points on the Earth's surface, astronomers were able to measure, with some degree of accuracy, the crucially important separation of the Earth and the Sun. Knowing this enabled them to convert the relative spacings of the planets indicated by Kepler's laws into absolute interplanetary distances expressed in miles or kilometres. Eli Maor's book presents the general reader with a full account of Venusian transits that covers the history of their observation as well as their significance and the reasons for their rarity. The book is a light and enjoyable read that opens well with an imaginative description of observing the 2004 transit from the hills outside Jerusalem. Following an account of Kepler's prediction of a transit of Mercury in 1631 and its observation by Gassendi, the book moves on to describe the transit of Venus in 1639, giving particular emphasis to the prescient work of Jeremiah Horrocks, the extraordinary young English curate and astronomer who died just two years later at the age of 21. The story, however, really takes off with Edmond Halley's realization, in 1677, that transits of Venus might provide the key to determining distances within the solar system. The details of Halley's method are confined to an appendix, but the central chapters of the book detail the increasingly elaborate efforts that astronomers made to observe transits of Venus up to the time of the 1882 transit, when, due to the impact of new photographic methods, interest in transit observations was waning. By that time the distance to the Sun was known to be about 93 000 000 miles and observations of Mars or the asteroids seemed to offer a better prospect of improved accuracy than further observations of Venus. The story is rounded off by a chapter that describes spurious transit observations (such as that of Vulcan, a planet that was supposed to orbit closer to the Sun than Mercury) and a chapter concerning transits of the Earth seen from Mars, Jupiter and the other outer planets. Maor, who is also the author ofe: The Story of a Number, and various other books, writes clearly and well, but Venus in Transit is not uniformly interesting throughout. The early chapters are generally very good, as is the account of Captain James Cook and theEndeavour's voyage to observe the 1761 transit. But after that I felt my interest flagging somewhat, just as the astronomers of the time seemed to find their own interest diminishing. The closing chapters helped to re-engage my interest, but I did feel that the discussion of the 19th century transits was rather perfunctory and that it would have benefited from more space. Still, these are minor criticisms of a book that I am personally very glad to have read. Venus in Transit will obviously appeal most to those with an interest in astronomy, particularly in its historical aspects. But there is also much that physicists can enjoy in the book and I can easily imagine it as a useful though non-essential addition to many school and college libraries. Robert Lambourne