The hunt for a peek at Venus's heavenly body
rOn Tuesday the planet can be seen crossing the sun from Earth. Charles Gris reports on the adventurers who have sought this rare sight over the centuries
Cast your eyes upwards and eastwards this Tuesday and you will witness an extraordinary event.
Venus, the nearest planet to earth, will cross the face of the sun between around 5.20am and 11.20am. And while it might not match an eclipse for drama, the scientific import of the occasion is huge - as is the rarity. There is not a person alive today who has witnessed such a Venusian spectacle.
Assuming the skies are clear, you will be able to witness something that governments have spent billions of pounds to observe. Hundreds of lives have been lost trying, sometimes forlornly, to measure such transits. One continent was discovered - at least by the rest of the world - as a result of a mission to observe one.
Most importantly, it is only because of earlier transits that we have, thoughout the modern age, had a reasonably accurate idea of our real place in the universe. It was that tantalising prospect that drove the Victorian adventurer George Forbes to the ends of the earth more than a century ago to watch Venus line up with the sun.
But to comprehend fully the passionate commitment of men like Forbes, you have to understand the magnetism of a unique astronomical phenomenon.
Transits of Venus occur in pairs every 120 years or so. The first recorded instance was in 1639, when Jeremiah Horrocks accurately predicted, and then observed, the phenomenon from a house near Liverpool. Thirty years later, Sir Edmund Halley was mapping the southern skies from St Helena, a tiny island 1,200 miles off the west coast of Africa, when he came to a profound realisation.
If a transit of Venus could be accurately measured from different points on the earth's surface, it would be possible, by the means of complex trigonometry, to calculate accurately the distance between Earth and the sun.
This idea gripped the imaginations of scientists and governments alike and by 1761, when there was next a transit, astronomers were stationed at more than 120 locations all over the world to take their measurements. The French and British navies battled - at times very bloodily - to obtain footholds on the remotest islands and the astronomers sailed for the best part of a year to get into position.
Guillaume Le Gentil, the French astronomer, was particularly unlucky. He spent four months at sea pursued by the Royal navy. He evaded them as far as Mauritius, only to discover that his intended observation spot, Pondichery, in southern India, was besieged by the British Army.
His hopes that the French navy might intervene on his behalf were dashed when its fleet hit coral reefs during a hurricane while on their way to India. Disaster hit again when his own ship was caught in storms, which meant that he was stuck at sea when the transit arrived.
Le Gentil was not the only one who failed. Many saw nothing because of poor weather. Others found that results taken from identical positions were at such variance as to be useless.
With those experiences behind it, the scientific community redoubled its efforts for the next transit in 1769. In Britain, the Royal Society commissioned Captain James Cook to sail with its observers, in a specially fitted out ship, Endeavour, to Tahiti, in the hope of taking accurate measurements.
Although cloudy skies in the days leading up to the transit augured ill, they cleared in the nick of time and the party from the ship took some of the best measurements that had yet been made. They then spent the next 2½ years "discovering" New Zealand and afterwards charting Australia's eastern seaboard.
Despite the quality of results obtained by Cook's party and others in 1769 the calculations that flowed from them were frustrating. For over a century astronomers crunched the numbers, but even the best attempts to measure the distance to the sun failed to add up. Such imprecision was never going to satisfy the grindingly empirical Victorians, so as the 1874 transit approached, fresh voyages were planned.
The scale of operations was already dizzying, as Forbes joined the fray as second-in-command of the British effort, bringing an extraordinary intellect to the challenge.
At just 22, after an impressive undergraduate career, Forbes had been appointed professor of natural philosophy at Andersons University in Glasgow (the nucleus around which Strathclyde University was created). Two years later the university granted a request from Sir George Airy, the astronomer royal, that Forbes be given leave to work on the transit observations.
Forbes left an astonishingly vivid account of the events which saw 27 professional observers and their crews travel to locations as remote as Egypt, New Zealand, the Kerguelen Islands and Hawaii.
In the spring of 1874, Forbes and Airy assembled their staff at Greenwich to instruct them in the use of the equipment they would be taking with them. This continued until June 3, when half of the staff sailed from Liverpool on the SS Illimani.
The long voyage gave Forbes the time to write a book about the forthcoming transit, explaining its significance and assessing the relative merits of the differing means of measurement.
After arriving in Hawaii on November 5, he immediately set about assembling the octagonal timber observation station. Once all the equipment had been installed in the 10ft x 15ft pod there was scarcely enough space for two people to work.
Everything was ready by November 15, and Forbes started making "continuous observations" to establish as accurately as possible their location and to determine any other factors that might effect the recording of the transit.
Forbes's notes from December 8, the big day, show how tense he was. "20.21. Clouds obscured the sun . . . I have almost given up hope." Nevertheless, by 8.34pm the clouds parted, and Forbes saw Venus at the start of the transit. Crucially, he was able to make a number of precision measurements with a micrometer attached to a telescope. He judged these results to be satisfactory.
Overall, however, the results did not render much improvement on previous figures. Airy laboured over his report for nearly a decade - paying for much of the work himself after falling out with the Admiralty over the length of time it was taking. Forbes, meanwhile, travelled back to Glasgow overland through Peking to St Petersburg, taking in the Gobi desert and Siberia - one of few westerners to have made that trip at the time.
Once back in Scotland, he made his name as a prolific inventor, and ended his days in the astronomical observatory he built for himself in Pitlochry.
The prodigious efforts of 1874 rather overshadowed those of the most recent transit, in 1882. Disputes over the veracity of the technique as a means of determining the distance to the sun meant that considerably less effort was devoted to new observations.
During the 1930s, the theory that had been applied to the transit of Venus was applied to asteroids, with far greater success.
And with the advent of radar (invented by another Scot, Sir Robert Watson-Watt, who lived in Forbes's observatory after the astronomer's death) a couple of decades later, the veracity of those results was established.
If you want to be one of those who observes the transit this week, some safety measures are necessary. To observe it directly, optical equipment of the quality of welder's goggles are necessary. Far easier, though, is to make a small pin-hole projector, or use a telescope to project an image of the sun onto a piece of card.
You might not be able to determine for yourself that the sun is 92,958,329 miles from Earth, but it will give you an insight into how we do know that.
Star Talks by George Forbes is published by the The Observatory Press at £4. For more information on George Forbes visit www.observatorypitlochry.com
Tim Dawson firstname.lastname@example.org
Telephone: 07984 165251
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