A transit of Venus is a rare astronomical phenomenon where an observer on Earth sees the planet Venus pass across the face of the sun. Due to the inclination of the orbits of the Earth and Venus, these transits come in pairs, eight years apart, with the pairs separated by a little over a century. They also always happen in June of December. Of course, it also must be daytime where you are when Venus crosses the sun. Just like the more common lunar and solar eclipses, Venus transits are only observable from certain places.
In order to know when a transit is going to occur, you have to have a very good understanding of the orbital dynamics of Venus. In order to see it, you need a telescope. Before the eighteenth century, both these resources were in short supply.
In 1639, the 21-year old astronomer Jeremiah Horrocks was studying Kepler’s Rudolphine Tables and comparing Kepler’s predictions of the motion of Venus with his own observations. Kepler had thought that Venus would just miss the sun in December 1639; Horrocks realized Kepler had made a mistake. Venus would transit the sun, and part of the transit would be visible (if the weather was good) from Lancashire, where he lived. Fortunately, Horrocks had bought himself a good telescope the year before, and the weather held. Horrocks, and his fried William Crabtree whom he had alerted, became the first people in all of recorded history to observe the transit. They also appear to have been the only ones. The image below shows the path Horrocks saw, bearing in mind that sunset that day was at 15:53.
Horrocks wrote up his observations in a Latin treatise called Venus in sole visa, and made important contributions to lunar theory before dying in early 1741 at the age of 22.
Having missed the 1639 transit, astronomers had to wait until the next pair of transits came along, in 1761 and 1769.
Apart from the obvious thrill of witnessing a rare event, why would astronomers care? The answer lay in the state of astronomy in the middle of the eighteenth century, and was again due to Kepler. Kepler had realized that planetary orbits in a heliocentric system were best modeled as ellipses, and his third law of planetary motion said that the square of the orbital period is proportional to the cube of the mean distance of the planet from the sun. Since astronomers knew the periods of the planets well, they could compute the relative distances of each planet, that is, relative to the distance of the Earth from the sun. But they didn’t know how far away from the sun the Earth was.
Although he was not the first to have the idea, the notion that a transit of Venus could be used to find the distance to the sun was popularized among the scientific community by Edmond Halley (he of comet fame). Halley himself would not live to see the next transits as he died in 1742, but he urged other astronomers to prepare. The basic idea was that observers in different places on Earth would see Venus make slightly difference tracks across the sun. This phenomenon, called parallax is similar to the way that if you hold a pencil at arms length and look at it with just your left eye, it lines up with a different spot on a distant wall than if you look at it with your right eye. The astronomers would be using the width of the earth as the two eyes. To have the best chance of success, the observing parties should be as widely separated as possible (subject to being in positions where the transits could be seen).
The key observations were the exact time the rear edge of Venus separated from the edge of the sun on entry, and the time when the leading edge of Venus just touched the edge of the sun on exit. So, the astronomers needed to know the exact local time, and their exact position, so that the times could be coordinated. Determining exact position in latitude and longitude was a major challenge and the details were not known for many locations. The observations were right at the limit of 18th century technology; the computations were long and difficult, and the prize was the size of the solar system. Thus was born the greatest international scientific endeavor of the century.
The leading scientific organizations of the day sought funds and volunteers, and there was a boom in instrument buying for the leading manufacturers, mostly in London. The French sent the Abbé Jean Chappe d’Auteroche to Siberia, Guillaume Joseph Hyacinthe Jean Baptiste Le Gentil de la Galasiere to Pondicherry in India and Alexandre-Guy Pingré to the island of Rodrigues in the Indian Ocean. The British dispatched Nevil Maskelyne to St. Helena and Charles Mason and Jeremiah Dixon to Sumatra. It wasn’t all plain sailing. 1761 was in the midst of the Seven Years’ War, with France and Britain on opposing sides. Before Le Gentil got to Pondicherry, it had fallen to the British, and Mason and Dixon’s ship was attacked by a French vessel shortly after leaving Plymouth. Badly damaged, it limped back into port for a refit that left the trip several weeks behind. In the end, Mason and Dixon only made it as far as the Cape of Good Hope, although they did make very good observations there.
As the reports trickled in from around the globe, it turned out there were problems despite the greatly improved instruments available. Good observations required accurate timing, but observers stationed at the same post reported the key events at different times, often by as much as 10 or 20 seconds. First, it turned out that Venus had an atmosphere and that blurred the image and so the timing; the second was that the separation and arrival of Venus at the edges of the sun were not clean simple events. Instead, Venus seemed to ‘stick’ to the sun for a while and again, that affected the timing of the separation. The observations did lead to improved estimates of the parallax and so the distance to the sun, but not to the level of accuracy that had been hoped for. It was time for the second push in 1769.
For the second transit, even larger and better equipped teams were sent to favorable locations. The Jesuit Father Maximilian Hell went to Vardø off the northern coast of Norway; Chappe swapped Siberia for (Spanish) California after months of tense diplomatic negotiations; William Wales was packed off to Hudson’s Bay; Le Gentil, who had failed to make any observations while stranded in the middle of the Indian Ocean in 1761 after the British captured his original destination in India, had spent the intervening time in and around the Indian Ocean before preparing an observatory in Manila although at the last minute the French sent him back to India; and Captain Cook was sent out to Tahiti.
Similar observational problems bedeviled the 1769 expeditions, but the results gave improved estimates of the solar parallax of around 8.7 to 8.8 arc-seconds, very close to the correct value. The solar system had been measured to an unprecedented degree of accuracy.
The stories of these expeditions make riveting reading. Sailing to the far ends of the earth meant absences for years in dangerous conditions; the Arctic observers had to overwinter in order to prepare for the observations the next year; the teams had to build their own observatories and contend with uncooperative locals; they had to make detailed observations over extended periods of time just to determine their exact locations. They were enormously dedicated, and years of effort could be wiped out by a single cloudy day. After 1769, none of them would ever have another chance.
The recent transits of 2006 and 2012 lead to a flurry of interest and there are a number of excellent websites with more information about the 18th century transits, and several books were published to coincide with the events. Two good ones are:
The Day The World Discovered The Sun, by Mark Anderson, and
Chasing Venus: The Race To Measure The Heavens, by Andrea Wulf, both from 2012.
Anderson’s book is a gripping read, with excellent use of primary sources to add spice to the narrative, and is particularly good on Father Hell. The description just of his trip from Prague to northern Norway takes 20 pages of colourful description. He is equally detailed in the other expeditions he focuses on, especially the two trips of Chappe, and Cook’s epic voyage to Tahiti. The price of the detail and focus is that he cannot include everyone. Wulf, on the other hand, goes for a more balanced approach, forgoing some of the detail to include many more expeditions. Hell leaves Vienna on p. 139 and arrives in Norway on the following page. Both books, however, will give you a good understanding of the motives of those involved and the perils, often mortal, they faced in search of a single more accurate number.
In case you would like to watch a transit yourself, the next one is due in December 2117, so you have plenty of time to get ready.