Danger and death in the name of science: How the 1761 transit of Venus across the sun revolutionised our view of the solar system



Transit begins at 5.55am BST on Tuesday morning for UK observers

Wednesday will be the last people living today to witness one of the rarest astronomical events: the transit of Venus.



If it is not too cloudy, observers will see the brightest star of the night transformed into a tiny black dot crossing the face of the sun.



The transit begins on Tuesday at 11.04pm - but as the sun has gone to bed in the UK at this time, it will be an early rise at 5.55am if you want to see the transit from here.



If you do grumble at getting up at this time, spare a thought for the astronomers in the 18th Century, who travelled thousands of miles and risked their lives to witness this precious sight.



Spectacle: A pelican and fishermen go about their routine on the Flagler Beach Pier while Venus creeps slowly across the rising sun They did so because they believed Venus held the key to the most pressing astronomical quest of the age: the size of the solar system. In 1716, British astronomer Edmond Halley had called upon scientists to unite in a project spanning the entire globe. He predicted that on June 6, 1761, Venus would traverse the burning disc of the sun for about six hours.

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Next The early Wednesday rise if you want to see Venus transit... Share this article Share If several people at different locations across the globe measured and timed this celestial rendezvous, they could calculate the distance between Earth and sun: the base unit for all distances in the solar system and the holy grail of astronomy. Visionary: Edmond Halley, a member of The Royal Society, predicted that on June 6, 1761, Venus would traverse the burning disc of the sun for about six hours The only problem was that transits of Venus only occur in pairs, eight years apart, but with an interval of more than a century before they are seen again. After this year, the next transit is not until December 2117. When Halley asked his international colleagues to rally, only one transit had ever been observed.

Knowing he would not be alive to orchestrate this global cooperation – a fact he lamented ‘even on his deathbed’ while holding a glass of wine – all he could do was place his trust in future generations. His gauntlet was taken up when hundreds of astronomers joined the transit project in the 1760s.

At a time when it took six days to travel from London to Newcastle, dozens of them travelled to remote outposts of the world to observe the phenomenon, laden with clocks, huge telescopes and other instruments. Many risked their lives. With the Seven Years’ War tearing Europe apart, they were even sent into war zones.

They made for strange adventurers: most of their lives were an endless round of dull routine, spending nights under the open sky or engaging in complex computations. The scientific world was electrified. The observations were the most ambitious scientific project ever planned, because the astronomers needed to watch the transit simultaneously from both the northern and southern hemispheres.

Venus’s path would be shorter or longer across the sun according to each viewing station. With the help of relatively simple trigonometry, the distance between sun and Earth could be calculated, but only if the astronomers shared their results afterwards.

Rare sight: Five transits of Venus have been recorded -- the last one priot to that in 2004 occurred in 1882 It was to be the first international scientific collaboration, and Europe was gripped in transit fever. Even Captain James Cook got involved, taking the Endeavour into the uncharted emptiness of the South Pacific The expeditions were organised by the scientific societies of Europe including the Royal Society in London. They sent Nevil Maskelyne, later Astronomer Royal, to St Helena – a lone speck of land in the South Atlantic.

With him he took an assistant, trunks full of instruments and more than 100 gallons of wine and rum. Charles Mason and Jeremiah Dixon, later famed for the Mason-Dixon Line, left Britain for Sumatra, but within four days they were attacked by a French warship.

The two refused to continue but the Royal Society threatened they would be prosecuted as mutineers ‘with the utmost Severity of Law’ unless they pressed on.

Some astronomers waded waist-deep through icy rivers; others saw their ships wrecked by tropical storms. On the day of the transit, June 6, 1761, about 250 official observers at more than 100 locations pointed their telescopes to the sky.

But instead of swiftly moving across the sun, Venus lingered at the edge for up to a minute, making exact timing impossible. They had one more chance, on June 3, 1769. Once again astronomers were dispatched across the world. Catherine the Great mounted eight expeditions across the vast Russian empire.

Astronomers from Britain, Austria, Switzerland and Russia travelled to the Arctic, where their brandy froze and clocks stopped.

Stunning: Earth's closest planetary neighbour appeared as a black disc 30 times smaller than the Sun's diameter, slowly moving from left to right over the course of six hours when it last appeared

The British even sent Captain James Cook and the Endeavour into the uncharted emptiness of the South Pacific to follow the transit from Tahiti.

Five astronomers died and many more faced unimaginable hardship. One French scientist travelled thousands of miles, facing bloody battles, dysentery and hurricanes only to be defeated by clouds.



When he returned home after 11 years, he had twice failed to see the transit, his heirs had declared him dead and divided up his estate, and he had lost his job.

It took years to collect the global data, but eventually the distance between Earth and sun was determined within a range of 92,900,000 to 96,900,000 miles – very close to today’s figure of 92,960,000 miles.

Impressive, but even more importantly, it was the beginning of international scientific collaboration.

On June 6 this year, the sun will rise just before 5am in Britain, and Venus will begin the show about an hour later.



But don’t look into the sun without proper protection. If you have sharp eyes, get some cheap eclipse shades or for a closer look, use binoculars or a telescope with appropriate solar filters.

But spare a moment for those astronomers who faced enemy attacks, deadly diseases and all kinds of adverse conditions when they united to measure the heavens.

Chasing Venus, by Andrea Wulf, is published by Heinemann, priced £18.99. To order your copy for £15.99 inc p&p, call 0843 382 1111 or visit mailshop.co.uk/books. This article first appeared on the Mail Online last month.

