GCSE Astronomy

The Transit of Venus on 8 June 2004

On this date, the planet Venus will be seen to pass in front of the Sun. This is actually a very rare event, occurring about every 120 years (the last one occurred on December 6, 1882). When it does occur it happens in pairs, so this year’s event will be accompanied by one in 2012.

The first observation of a transit was on 4th December 1639 by Jeremiah Horrocks, originally of Toxteth Park, Liverpool, but who probably carried out his observations at Hoole, east of Southport. Horrocks had calculated the time of the transit himself, correcting the planetary data in Kepler's Rudolphine Tables. These new tables from Kepler had proved their worth in predicting a transit of Mercury, which had been duly observed by Gassendi in 1631.

The transit started just around 1500, allowing observations for about an hour before the Sun set.

Later, it was realised by Edmond Halley that accurate observation of a transit would allow calculation of the distance between the Earth and Sun (actually Halley might have extended the ideas of J. Gregory from 163). This had the effect of increasing interest considerably.

So for the event of 1761, about 200 astronomers were geared up, including the British astronomers Mason and Dixon (well-known names in a different context, and the subject of a song by Mark Knopfler) who were due to observe from Sumatra. This was the time of the 7 Years War and their ship was attacked by the French only a few hours after leaving Portsmouth, leaving dead and wounded. After this experience, they were loathe to continue but the Royal Society threatened them with ruination if they did not proceed. As at least one observer has noted, their fear of the Royal Society was greater than that of the French, and they re-embarked. In any case, by now it was too late to reach their original goal – but they did actually manage to make the Dutch Colony of Capetown , from where they observed the transit.

Nevertheless, no great accuracy was claimed for the Earth-Sun distance, leading to even greater interest in 1769. In 1768, Captain Cook left Plymouth in the Endeavour bound for Tahiti, which had only been discovered by Europeans the year before. His express task in Tahiti was to observe Venus and Cook reported that the astronomical work was a success, While he was there, Cook noted the apparent ease with which the Tahitians obtained their daily nourishment — needing only to pluck the fruit of the bread fruit tree. (An observation leading to well-known consequences later. Note that although Bligh did sail with Cook, that was on a later voyage). Unfortunately, the official astronomer, Charles Green, died, along with many of the crew, before reaching home.

The unfortunate figure of Jean Baptiste Le Gentil, from Normandy, is always mentioned in connection with these transits. He had travelled to Pondicherry in India for the 1761 event but was unable to observe because the British had occupied the town. He decided to stay in the East another eight years for the next transit. On the day, cloud appeared shortly before the event and spoiled his view again. Eventually arriving back in France, he found his relatives had thought him dead and sold all his property.

Despite Captain Cook’s optimism, the Earth-Sun distance still remained undetermined, so the transits of 1874 and 1882 still attracted the interest of many astronomers. The distance calculated this time round was accurate to within 200,000 km (from a total distance of 150,000,000 km).