The Transit of Venus

3D front cover of Memoir of the Life and Labors of the Rev. Jeremiah Horrox edited by Arundell Blount WhattonIt’s always the same thing – you wait for ages for a once-in-a-lifetime moment, and then three come along at once:  the London Olympic Games next month (if you like that sort of thing), the Diamond Jubilee celebrations of Queen Elizabeth II this weekend, and the transit of Venus, on 5 June.

That part of the eternal music of the spheres which determines that Venus crosses the face of the Sun twice in an eight-year period (either once in 121.5 years or once in 105.5 years) was first brought to the attention of British science in 1639, by Jeremiah Horrox. The significance of the astronomical event for calculating the distance of the Earth from the Sun, and hence the size of the solar system, was appreciated, not least by the Royal Society, and preparations were put in train for international collaboration to make observations at the next transit. As a result, for the second of the two events of 1761 and 1769, Lieutenant James Cook was given command of H.M. S. Endeavour, with instructions to find an appropriate location in the south Pacific from which his scientist passengers could observe the transit.

On 3 June 1769, Cook, Joseph Banks, Daniel Solander and the astronomer Charles Green made observations from three points on Tahiti (Solander’s telescope was apparently more powerful than Green’s officially provided one). They had set up a fortified camp (‘Fort Venus’) on a sandspit on the shore (the rocking ship would have been no use for accurate observations), and after certain alarms – including the theft of the wooden case containing the astronomical quadrant which was key to the enterprise, and which Banks managed to retrieve – and a sleepless night brought on by anxiety over the weather, they were rewarded with ‘the sun’s rising … without a cloud’, and ‘the whole passage of the planet Venus over the sun’s disk was observed with great advantage’.

Unfortunately, when the Endeavour returned from its great voyage (Cook had further sealed orders to explore the southern Ocean and the coasts of Terra Australis), the Royal Society was very disappointed in the data obtained. Green (who had died on the return journey) was blamed for inaccuracy and carelessness, but in fact all observations worldwide suffered from a similar problem (a haziness which made timings difficult and was later recognised as distortion created by the Earth’s atmosphere). Cook defended himself and his team with vigour, and in fact the consequent calculation, by Thomas Hornsby in 1771, of our planet’s distance from the Sun (‘93,726,900 English miles’) is only slightly different from that derived from the most recent estimates (92,955,000 miles).

The consequences for Britain and for the world of the Endeavour’s voyage were, as they say, game-changing. The same cannot exactly be said of the scientific expeditions to observe the next transit of Venus, which took place in 1874 (with the second event in 1882). But in 1879, the Royal Society published the ‘non-astronomical’ results of two of the voyages organised by the British government, which visited two remote and intriguing locations – Kerguelen’s Land, or the Desolation Islands, in the southern Indian Ocean, and the island of Rodriguez (these days spelled with a final ‘s’), a long way east of Madagascar.

Both places were relatively unexplored. The Kerguelen Islands were four thousand miles away from South America, but shared the same species of flora; while Rodrigues, unlike most archipelagic islands then discovered, which were volcanic in origin, was formed of limestone, and moreover, most of its local species were not indigenous. Both locations provided some of the first clear evidence that modern sea-levels were much altered from those of prehistory.

H.M.S. Challenger visited Kergulen’s Land with the Rev. A.E. Eaton as its naturalist (the islands had previously been explored by James Clark Ross during his 1839–43 voyage, during which the on-board naturalist had been the young J.D. Hooker, who was one of the prime movers of the 1879 publication). Eaton made observations and collected samples which were then examined and classified by botanists and zoologists including Eaton himself, Hooker, and Sir John Lubbock (who was an entomologist when he wasn’t being an expert on early Man). Rodrigues was visited by H.M.S. Shearwater, with Isaac Bayley Balfour, assisted by Henry Slater and George Gulliver, as naturalists.

On the whole, the Rodrigues team may have had a better time. The Desolations Islands were not so called for nothing, and Kerguelen has (as far as is known) never been permanently settled. It used to have a whaling and sealing station, and today holds a French scientific base: the climate, according to Eaton, is ‘tempestuous, chilly, and wet. Days perfectly calm are of extremely rare occurrence, gales, or at least strong breezes, being almost constant’. The soil is so boggy and treacherous that land-based expeditions are both difficult and dangerous, and there are no trees. (Sadly, according to a French blog about the islands, the indigenous smaller wildlife these days is under threat from the 6–7,000 feral descendants of two or three cats introduced into the scientific station in the 1950s: there is an unpleasant picture of a sheathbill (Fr. chionis) after a one-sided encounter with one of these predators.)

In Rodrigues, there is a risk of hurricanes in season, but the climate is tropical and the island was at one time verdant: François Leguat of Brest (to whose account the scientists refer) described it in 1691 as a ‘little Eden’. But again, human interference, in the form of introduced goats, had a drastic effect on the ecology of the island: the protective layer of foliage having been chomped away, soil erosion followed, and the remaining dry scrub was often further degraded by bushfires. (These, happily, seem to have become rarer: a video by the present-day tourist board shows idyllic beaches, clear blue seas, birds,  flowers, boating, swimming, diving, sea food and a wedding on a beach …)

This time, the entire transit will be visible from the western Pacific: in western Europe, our opportunity begins at sunrise on 6 June, by which time the event will almost be over. Go to the Greenwich Royal Observatory website, where, inter alia, you can learn that the Kew Observatory was built by George III so that he could observe the 1769 transit from the comfort of his own palace. What may ruin it is of course the unpredictability of the weather (see the Thames Jubilee Pageant yesterday): though again, the world-wide web will save us. If you let these people know where your computer is located (in my case they’re six houses out, but I won’t quibble), they will tell you what time in the morning you have to get up , and will also provide cloudy/wet-weather alternative things to do. It’s a far cry from Cook and his companions, setting off into the unknown.

Caroline

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This entry was posted in Astronomy, Earth Sciences, History, Life Science, Mathematical Sciences, Physical Sciences, The Naval Chronicle, Travel and Exploration and tagged , , , , , , , , . Bookmark the permalink.

4 Responses to The Transit of Venus

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