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Portrait of David Gill by George Henry, 1912, oil on canvas.
Photoheliography was abandoned for the British efforts of 1882. Again, international expeditions travelled to the best possible observing points: the British sent groups to the West Indies, Australia, Madagascar and New Zealand, as well as to South Africa. Here, David Gill FRS (1843-1914) co-ordinated multiple observing parties while viewing the transit himself from the Royal South African Observatory in Cape Town. Gill had been unfortunate in 1874: as part of a private expedition to Mauritius, he had a poor view. But as the 1882 transit approached, and using the instruments set up for the event, one of Gill’s assistants saw what became known as the Great Comet of 1882 and Gill himself photographed it – it must have seemed a good omen and so it proved.
Gill’s dispersed teams had clear skies and produced over fifty ingress and egress measurements and these could be reduced to produce a value for the solar parallax. But Gill himself was dissatisfied with the method generally and the accuracy of the results obtained using Venus. At Ascension Island in 1877, accompanied by his wife, Gill had taken Earth-Mars parallax measurements during Mars’s opposition in the morning and evening – using the diameter of the Earth as a baseline as the planet turned beneath him. By employing Kepler’s Law, Gill could proceed to estimate 1 astronomical unit His results were more accurate than the transit method, since there were none of the “black drop’ observational difficulties. Despite the many 1882 Transit of Venus observations, including David Gill’s own, the solar parallax method had become outdated. Gill had, using a different method, solved “the noblest problem in Astronomy”.
The 1882 transit produced less popular and scientific excitement than formerly. Perhaps rapid communication had taken some of the romance from even this rare astronomical display and there was certainly less adventure than in James Cook’s day. The nature of its scientific value had changed too. 1882 would be last occasion on which major expeditions were organised to solve a problem that, in previous generations, had consumed people’s lives. The most famous casualty of Venus was the French astronomer Jean-Baptiste Le Gentil (1725-1792), who, in 1761 was approaching the French colony of Pondicherry in India, his proposed observing station, only to find it overrun by the British. Forced to turn back, the transit occurred while he was still rocking aboard ship, an impossible viewing platform. Rather than return to Paris, he determined to stay in the East until the 1769 transit. But after eight years, all Le Gentil saw was a cloudy day and upon reaching Paris in 1771 he discovered that he had been declared dead: and had been ejected from the French scientific academy.
1639 | 1761 | 1769 | 1874 | 1882
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