Enfilade

The last transit of Venus was in 2004, but the next one won’t occur until 2117. From Captain Cook’s first voyage to the construction of the Kew Observatory, these celestial events were enormously important in the eighteenth century: 1761 and 1769 (click on either date for details at the Royal Society’s website). For a sense of just how important, see the eighteenth-century bibliography compiled by Utrecht’s Institute for History and Foundations of Science. -CH

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From Royal Museums Greenwich:

Measuring the Universe: From the Transit of Venus to the Edge of the Cosmos
Royal Observatory Greenwich, London, 1 March — 2 September 2012

Royal Observatory, Greenwich, photographed on 24 July 2006 (Wikimedia Commons)

On 5-6 June 2012 a very rare astronomical event takes place: the planet Venus will pass directly between the Earth and the Sun, appearing as a small black dot against the face of our parent star. These transits of Venus occur in pairs eight years apart, but each pair is separated by more than a century. The last one was in 2004 and, after June 2012, the next won’t be until December 2117.

Historically, transits of Venus were used by astronomers to give the first accurate measure of the distance between the Earth and the Sun. To be sure of observing these twice-in-a-lifetime events expeditions were sent around the world in the 18th and 19th centuries and the story involves many famous characters. Captain Cook was sent to Tahiti to observe the transit of 1769, and King George III had Kew Observatory built so that he could view the transit himself (the telescope he used will be on display in the Royal River exhibition at the National Maritime Museum).

By the 21st century the distance to the Sun was well-established, with confirmation by radar studies and space missions. But the 2004 transit still excited the interest of the press and public and was even the subject of a series of photographs by Turner Prize-winning artist Wolfgang Tillmans, some of which can be seen in the Royal Observatory’s free exhibition Measuring the Universe. And now the idea of transits has acquired a new significance for astronomers as they are used to discover new planets orbiting distant stars.

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As Paul Cockburn writes for Time Out London (28 May 2012) . . .

2004 transit of Venus taken from the Royal Observatory

‘This one really is your “last chance to see”,’ explains Dr Marek Kukula, public astronomer at the Royal Observatory, Greenwich. ‘The next one is not until December 2117 – very few, if any, people alive today are going to see that.’

‘When watching a planet slide in front of the Sun, the mechanics of the heavens are laid open for anyone to see,’ Kukula says. And, as the exhibition Measuring the Universe, currently running at the Royal Observatory, explains, the transit of Venus has played an important historical role in helping astronomers calculate the size of the solar system.

‘In the eighteenth century, the transits were among the first examples of international big science collaborations,’ says Kukula. ‘Despite a lot of national tensions within Europe, scientists were communicating with each other across borders. These were also early examples of government-sponsored science; Captain Cook’s first voyage to the South Seas was funded so that he could observe the 1769 transit from Tahiti.’ On the way back, of course, he went on to discover New Zealand and Australia.

There were practical considerations in all this. ‘By measuring the transit accurately, you gained a new level of understanding in how the heavens worked, and that actually was very useful for navigation,’ says Kukula. ‘Even in the eighteenth century, though, the observations were not quite good enough to really nail it. For the nineteenth-century transits – in 1874 and 1882 – you have Greenwich sending out numerous expeditions around the world. Armed with photography, they can at last make accurate observations.’ . . .

The full article, with directions for safe viewing, is available here»