On Saturday 2nd of June I am leaving for a trip to the Gobi desert. The reason I am heading off there is to observe the Transit of Venus. The June Transit on the 6th will be the last one until Decemeber 2117 – in fact transits of Venus are the rarest predictable astronomical event we know of.
During a transit, Venus appears to pass across the disk of the Sun. Venus passes the Earth in its orbit every 584 days, something we call an inferior conjunction. But, because the planes of orbit of the two planets around the Sun are inclined to each other at 3 degrees, Venus will only appear to pass across the disk of the Sun on the rare occasion when an inferior conjunction happens when the two planets are on the line of nodes – the line where the two planes cross.
Kepler was the first person to predict a transit of Venus. In 1629, after he had worked out his 3 laws of planetary motion, he calculated that in December 1631 Venus would appear to pass across the disk of the Sun. He also calculated that Mercury would transit in November of the same year. The November Mercury transit was observed by Pierre Gassendi in Paris, and Jesuit Father Cycat in Innsbruck and Johannes Remus in Alsace. The only surviving sketch we have is from Gassendi.
Gassendi also tried to observe the December transit of Venus, but failed. We now know that the Transit of December 1631 was not visible from Paris. Kepler had predicted that the next Transit of Venus would be in 1761, but in fact he got his calculations wrong. In 1639 the young English astronomer and mathematician Jeremiah Horrocks calculated that Venus would transit across the disk of the Sun in December of that year, 8 years after the transit of 1631. He wrote letters to his friend Crabtree in Manchester, and the two of them became the first human beings we know of to observe a Transit ot Venus. In fact, Horrocks kept a detailed journal of the observations.
At this point transits of Venus were just a curiosity. But this all changed in 1715 when Edmund Halley presented at paper at the Royal Society where he showed that transit of Venus could be used to measure the distance from the Earth the Sun, a distance which had eluded all attempts to be measured up to this time.
The method Halley proposed depended on the effect of parallax. If two observers were to observe and time the Transit of Venus from different locations on Earth, separated by latitude, then the difference in the path length of Venus across the disk of the Sun, if one knew the distance between the two observing stations, could determine the distance from Venus to the Sun and hence the distance of Earth from the Sun.
Halley also knew that he would not live to see the next pair of transits in 1761 and 1769, but his admonishment was remembered, and in 1761 an international effort was made to observe the Transit of Venus in order to finally determine the distance from the Earth to the Sun. As (bad) luck would have it, altough nearly all the scientists were based in Europe, the Transit could only be seen in its entirety in Asia, southern Africa, and the northern parts of the Northern Hemisphere.
Data for the 1761 were obtained from 60 different observing stations in 8 countries, making it at the time the largest international science project ever undertaken. There were a number of problems in the timings of the 1761 transit times from the various locations. This was due to the so-called “black drop” effect, which no one expected. It led to errors in the contact times between Venus and the disk of the Sun, rendering much of the data gathered useless. This led to the 1769 Transit gaining importance, as astronomers knew it was their last chance until 1874 to observe a transit, and with fore-knowledge of the black-drop effect, they hoped their data would be less error prone.
The 1769 Transit led to Captain Cooke going to Tahiti, where he and his team set up an Observatory. They made crucial observations near the Halleyan point, the point on the Earth where the Transit would appear to be the longest.
Due to the astronomers for the 1769 Transit knowing about the black-drop effect, the timings were far more accurate. About 160 scientists made observations from over 70 different observing stations. In 1771 the the data were complied by Thomas Hornsby, Professor of Astronomy at Oxford University, to finally determine the distance from the Earth to the Sun, a calculation which astronomers had been trying to do for thousands of years. He determined the value to be 93,726,900 miles (this compares extremely well with the currently accepted value determined by RADAR which is 92,957,133 miles).
There was a pair of transits in 1874 and 1882, and 8 years ago, in 2004, Europe was lucky enough to be well placed to see the entire Transit. I led the organisation of a public observing event in South Wales, and from tables published by NASA I was able to see that it was the first Transit visible in its entirety from Wales since 1283, and the next one visible in its entirety from Wales will be in 2247. Unfortunately for Europeans, the 2012 Transit requires another trip. As the map below shows, one has to be in the Pacific Ocean area of the World to see the 2012 Transit in its entirety. That is why I am going to the Gobi, in the hope that the only desert in the Northern Hemisphere that is in the region to be able to see the entire Transit will give me clear skies. I don’t expect to be around for the next one in 2117!