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Book Review, The Transits of Venus |
Posted by Michael Cohen, Jun 29, 2004
The Transits of Venus, by William Sheehan and John Westfall (Prometheus Books, 2004). 407pp. 1-59102-175-8. $28.
Not long ago, I stood on a North Carolina Beach just after dawn and watched the end of Venus's 2004 transit of the sun. A few hours before, no one alive had ever had the opportunity to observe a transit of Venus, because the last one had occurred 122 years earlier. Now millions have seen it. My fellow watchers of the June 2004 transit were interested in the spectacle of a near-Earth-sized planet crossing the sun. Unlike the earlier transit-watchers described in Sheehan and Westfall's book, we were not doing measurements that would alter the fundamental numbers that describe our solar system.
Sheehan and Westfall succeed in conveying why the eighteenth-century transits of Venus were such important scientific events. They give a fairly clear explanation of how astronomical measurements build on one another. The size of the earth, for example, can be determined by applying a little spherical geometry to the comparative length of shadows cast by objects of the same height in cities of known distances apart. Once we know the earth's size, we can use it to determine the distance to the moon by measuring the moon's position relative to the stars from different points on the earth or from the same point at different times during the earth's rotation. The sun is too far away for this kind of measurement, but we can ladder our measurements until we know the size and distance of the nearer planets to a fair degree of accuracy.
A transit of Venus is a relatively rare phenomenon, occurring twice about every hundred and twenty years, when the planet Venus moves across the face of the sun. Astronomers in the eighteenth century theorized that such a transit would allow them to apply a known quantity, the size of Venus, to make a precise measurement of the solar parallax, the angle formed by imaginary lines drawn from the sides of the earth toward the sun. When we know the diameter of the earth and the angles of the triangle thus formed, we can calculate the distance to the sun. The earth-sun distance, which we call the astronomical unit or AU, we now know to be approximately 93 million miles. This number in its turn enables us to go outside the solar system and begin measuring the distance to other stars.
Before astronomers could even predict when transits of Venus would occur, the Copernican, sun-centered model of the solar system had to supplant the older Ptolemaic, earth-centered model. Then it became obvious that the "inferior planets" of Mercury and Venus, being closer to the sun than we are, could occasionally cross its face in the passage known as a transit. And so the first transit of Venus that was observed was not until the seventeenth century. Two Englishmen, Jeremiah Horrocks and William Crabtree, became the first people to witness a transit of Venus in 1639. But not until the eighteenth century transits of 1761 and 1769 were all the elements in place for precise observation. Expeditions mounted by half a dozen European countries provided what was necessary: precise timings of the transit made from places of widely differing latitudes. Observers included Benjamin West, the painter, who watched the 1769 transit from Providence, Rhode Island, the surveyors Mason and Dixon, who saw the 1761 transit from Cape Town, and, observing from Tahiti in 1769, Captain James Cook, whose party included the naturalist Joseph Banks and the Scottish landscape painter Alexander Buchan.
Although the transits of 1874 and 1882 refined the precision of the solar parallax value, they also proved that, even with photography, the transit method of determining solar parallax would never be as precise as everyone had hoped. Such measurements have since been superseded by more accurate ones such as determining asteroid orbits and bouncing radio waves off planets.
The saddest story about any of the transits is that of Guillaume Le Gentil, who left France to travel to India to observe the 1761 transit. France was at war with England, which laid siege to Pondicherry, Le Gentil's destination on the subcontinent, while the Frenchman was en route. He hesitated, and then arrived too late to see the transit. He decided to stay in India until the 1769 transit. Clouds prevented him from seeing any of it. After some further misadventures on the way home to France, Le Gentil arrived to find he had been declared dead and his estate divided among his heirs. Le Gentil's sad story was a cautionary tale I and other prospective transit watchers repeated to each other to prepare us for the chance of being frustrated by bad weather.
Sheehan and Westfall provide a good deal of background to other astronomical questions of the centuries they describe, such as the attempt to find a reliable method of determining longitude. They try, not always successfully, to tie developments in astronomy to geopolitical events.
This book was published shortly before the 2004 transit. Sheehan and Westfall give detailed directions for observing the transit, and for both the 2004 and 2012 transits they provide world maps that show how much of the transits' duration can be viewed from any point. Tables at the end of the book give information for more than 150 cities worldwide, including the times and sun altitudes for each contact of the planetary and solar disks as well as the percentage of sunshine hours for these cities in June.
There are some typos and errors (the 1761 transit is given in one table as occurring in 1701, for instance), but Sheehan and Westfall, who provide notes for their sources, a bibliography, and a good index, write clearly for the most part. I say "for the most part" because nowhere in the book do they explain how timing the transit of Venus from two widely spaced points on the earth allows us to calculate the solar parallax. But Sheehan and Westfall are successful in telling the human stories of those who ventured reputations and lives to learn the distance to our nearest star.
