William Sheehan (1) (1954–)

When I was growing up, I wanted to be a “naturalist”, which I defined as somebody who went to faraway places and did neat stuff that had something to do with science.. I devoured the writings of Roy Chapman Andrews and William Beebe and Howard Carter, the “...for Sam” books, the Golden Nature Guides, and anything else related that I could drag home from the public library. Alas, the west Chicago suburbs were singularly unconducive to natural history; West Towns Transit did not run buses to Outer Mongolia, I couldn’t persuade my parents to buy me a bathysphere, despite attempts to dig one up in the back yard I found no pharaonic tombs, and none of the exotic wildlife illustrated in the nature guides ever put in an appearance on East Adams Street. Thus I had an epiphany when I eventually discovered astronomy; you can do it from your back yard - especially if you shoot out annoying nearby streetlights with your BB gun.

Even so, some of the more interesting astronomical phenomena require a little travel. I’ve been to six solar eclipses - prior to the 2017 one the nearest one to home was in Miahuatlan, Mexico. And I’ve seen a transit of Venus from the roof of a hotel in Luxor, Egypt. This book is about people who had to put up with somewhat more discomfort to do the same thing. Once Kepler formulated his laws of planetary motion, it was quickly realized that if you found the distance between the Earth and the Sun you could quickly work out the distances to all the other planets. And one way to do this is to time a transit of Venus from a known position. Unfortunately, the orbital relations of the Earth and Venus make this a little tricky; transits take place in a cycle of 105.5 years, 8 years, 121.5 years, and 8 years. Thus it’s an accident of birth if a transit takes place in your lifetime.

Although Kepler correctly predicted a transit in 1631, nobody saw it; it was cloudy all day in Europe and nobody else was interested. Kepler missed the fact that there would be another transit in 8 years, so it fell to an amateur mathematician, Jeremiah Horrocks, to work out that there would be such a transit, that it would be visible from England, and he better get going if he wanted to see it because he worked out the math in late October 1639 and the transit was due on December 4. The day was clear; he set up his telescope to project an image of the Sun, and became the first known human to watch the little black dot crawl across the solar disk. He didn’t get much fame from it, and was dead two years later at the age of 22.

However, by the time the next transit pair rolled around the whole world was watching - or trying to. The dynamics of transit observation made it necessary to make observations as far apart in latitude as possible, so for the 1761 transit the European powers (nobody else was interested, yet) sent expeditions to Siberian and the Indian Ocean. One assumes the short straws went to Siberia. The fact that most of the hopeful transit watchers were French or English was complicated that France and England were at war at the time. Mason and Dixon (the line guys) headed for Sumatra, found that the French had seized their intended observing site, and decided to make the best of it by camping out at Capetown. The French astronomer Guillame Joseph Hyacinthe Jean Baptiste Le Gentil de la Galasiere (may his name live long) had reverse luck; his site in India had been captured by the British. He set out anyway in the hopes that it would be recaptured by the time he got there, got a run of unfortunate weather, spent months tacking and wearing around the Indian Ocean, and spent transit day uselessly on the heaving deck of a ship. Well, what’s 8 years to an astronomer? La Gentil decided to stay on, idled around identifying plants and bugs and fish and other naturalist type things, and eventually set out for Manila in 1769. He almost got shipwrecked in the Strait of Malacca, turned around, and eventually headed back to India. After days of fine weather, it clouded over on transit day. Cleared up eventually - after the transit was over. Eight years for nothing. Other observers were luckier, although they had their problems too. Natives with culturally different ideas about personal property stole everything that wasn’t nailed down from Captain Cook in Tahiti, then stole the nails. The expedition to Hudson’s Bay got there the previous fall, spent a winter so cold their brandy froze, and were almost vampirized by mosquitoes. A French expedition to Baja California ran into a typhus epidemic and lost 75% of their party.

By the time the 1874-1882 transit pair rolled around, astronomers had begun to realize that the Venus observations weren’t as easy as they looked. The "teardrop effect" made accurate timings difficult. Plus other ways of determining the Earth-Sun distance had been developed. So this was transit astronomy’s last hurrah. Once again, astronomers spread out all over the earth, and this time other countries got into the act - the United States, Germany, Mexico, Japan, Egypt and Argentina sent observers to one or both transits. The strangest story from this one is a love triangle between Austin Dickinson (Emily’s brother), Mabel Loomis Todd , and David Peck Todd, professor of astronomy at Amherst and her husband. Apparently all parties involved acquiesced to the menage a trois, with Austin and Mabel observing from Massachusetts and David heading to California.

Transits are just an astronomical curiosity now; you get much better results from radar. Still, it was quite a thrill in 2004 to watch something no one had seen in 112 years.… (meer)

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… (meer)