Notes from Under Sky

Venus in Transit

In which I review this book

Title: June 8, 2004: Venus in Transit, by Eli Maor
Price: $22.95 (list), $16.06 (Amazon)
Published 2000 by Princeton University Press
ISBN 0-691-04874-6, 200 pp.

In roughly four years, an astronomical event will pass that takes place merely twice every 120 years or so. From a visceral point of view, this event is not nearly the most striking: a tiny dot passes across the face of the sun over the span of several hours. Even a lunar eclipse is more interesting, and a total solar eclipse is simply stunning by comparison. And yet, this event is in some ways more special still, for its rarity and its history. On 8 June 2004, as seen at least in part from over half the globe, Venus will transit the sun.

Eli Maor's book is not just about the mere event itself, to which he devotes just one chapter and snippets of others. He gives us the history and the science behind the six previously observed transits, dating back to three observations of the 1631 transit. In particular, transits have been used to determine a variety of solar system parameters, such as the diameters of Mercury and Venus. None of these, however, was as important as the length of the astronomical unit (AU), the mean distance between the sun and the earth.

Edmond Halley, who first determined the orbit for the comet that bears his name, had also developed a means to determine the length of the AU from transits of either Mercury or Venus. Two observers could watch a transit from two different locations on the earth. Each one would see the transit from a different perspective, and would therefore time the transit differently, much as your left and right eye see slightly distinct views of the same scene. The exact difference between the two observations would give us the distance to the sun.

Transits of Mercury occur much more commonly than those of Venus, so it would seem at first that Halley should have developed his method (which was full of complex trigonometry and took years to write out in detail) for the innermost planet. But Mercury, it turns out, is *too* close to the sun. The two observers would see little difference between their views. Despite its less frequent crossings, Venus was a better candidate, because at its closest, it is only about 40 million kilometers away from the earth, whereas Mercury gets no closer than about 85 million kilometers. Therefore, Halley carried out his calculations for the pair of transits that would occur in 1761 and 1769, taking place, he knew, after his death.

Maor has appendices to describe in greater mathematical detail the method that Halley arrived at, as well as predictions for future transits. The main point of the book, however, is the human element in observing transits—the desire for fame (though scant fortune) and new learning.

Maor describes the human drama of observers battling the elements and circumstance to observe the two transits. Probably the most heartbreaking story is that of the Frenchman Le Gentil (1725–1792), who planned to observe the 1761 transit from Pondicherry, in India. Unfortunately, before he could arrive, Pondicherry was taken by the British, who of course had no desire to welcome Le Gentil. So the ship turned around and the hapless astronomer was reduced to observing the transit from the boat, where accurate timing was impossible. He decided to stop in Mauritius and stay for the eight years before the next transit, spending the time observing the local flora and fauna. He set up to observe the 1769 transit from Pondicherry again, where the British were now directed to aid him, but on the fateful day, a single cloud hid the event from his eyes.

Others fared better than Le Gentil, but they were daunted by another unexpected problem: Venus's thick atmosphere caused the exact moment of ingress and egress to be blurred and indistinct, an aberration called the "black drop effect." This made precise timings nearly impossible, and caused an error bar of a few percent in the AU. That may not seem like much, but the AU was already previously known within about 10 percent, using a method costing much less in equipment, travel, and effort.

In fact, that Maor omits this second method is one of the few problems I have with the book. In 1672, Cassini and Richer used observations of Mars, which could be carried out every two years or so, near Martian opposition, to determine the AU at about 138 million kilometers. The true value is about 150 million kilometers, so the accuracy bar was already set fairly high. Knowing this makes the imprecision of the transit method even more disappointing.

Overall, though, Maor's book makes fascinating reading. It should be accessible, with some effort, to most readers of this newsgroup, but that effort—unlike Le Gentil's ill-fated attempts—will be amply rewarded.

Copyright (c) 2000 Brian Tung