Veni, Vidi, Venus

On 6 June, for six and a half hours, a dark speck will blemish the clear, bright surface of the Sun. It will be Venus, Earth’s nearest neighbour, making one of its infrequent passages across the face of our star. Such crossings occur in a tricky pattern along a 243-year timeline: pairs of transits just eight years apart are themselves separated by gaps of more than a century. The last event took place in 2004; miss this one, and you’ll be waiting until 2117.

But beyond their stutter across the astronomical calendar, these transits have a particular cultural resonance in the history of British Australia. The primary purpose of James Cook’s 1769 Endeavour voyage was to observe that year’s Venusian event from Tahiti; by the time of the 1874 transit the head of Sydney Observatory, Henry Chamberlain Russell, was championing the colony’s own observations.

“The idea was to observe from as many places as possible,” says Nick Lomb, the former curator of astronomy at Sydney Observatory and author of the splendidly illustrated Transit of Venus: 1631 to the Present. “But in the eighteenth and nineteenth centuries, it was also a matter of prestige. That’s how Russell sold it to the NSW government: great for science and great for the colonies to be involved in this huge international effort.”

Transits merited such attention because they provided a rare opportunity to measure the distance between Earth and the Sun – the baseline number in almost all other astronomical calculations. In 1716, Edmond Halley – of comet fame – had suggested that by timing Venus’ passage as seen from different locations, it would be possible to make that calculation. In 1769, Britain had observers in Canada as well as Tahiti; the Dutch in Batavia (now Jakarta); the French in Baja California. A Hungarian priest (the irresistibly named Father Maximilian Hell, then head of the Vienna Observatory) even travelled to Vardø, in far northern Scandinavia.

But Halley’s simple-sounding suggestion ran into the unexpected difficulty of the ‘black drop effect’ – the apparent smearing or distortion of the edge of Venus as it touches the edge of the Sun. This makes it very difficult to pick precisely when the transit begins and ends. (The illusion is probably caused by a combination of the Sun’s gaseousness and what one astronomer calls the “inherent blurriness” of any optical viewing system.) And then there was the problem of collating the data. Cook’s expedition took two years to return home, and others suffered far worse fates: the French team was almost entirely wiped out by disease.

Now, of course, that crucial span between planet and star has been measured by radar: it’s 149.6 million kilometres. But the transit’s significance has not entirely disappeared. “If you’d asked me in 2004 if it had any scientific importance, I’d have said no,” says Lomb from his home in Melbourne. “But in 2012, it has serious, cutting-edge relevance again.” The Kepler spacecraft, launched in 2009, has been using the transit technique – observing a dip in a star’s brightness as an object crosses its face – to hunt for Earth-like planets orbiting other stars. (More than 2300 possibilities were spotted in its first 16 months.) “The next challenge is to understand more about these planets,” says Lomb, “and the best model we have for that is Venus’ transit. Astronomers will use this transit to try to analyse Venus’ atmosphere and take other readings that can be applied to these new discoveries.”

Perhaps more excitingly, the advances in personal communications technology in the last eight years mean that anyone with a smartphone can re-create the work of Cook and his astronomer in 1769. “There’s an app,” says Lomb, “which will allow the public to measure the distance between the Earth and the Sun. It’s not scientifically important, but it’s a tremendously important public project.”

The app is the brainchild of Steven van Roode, a 36-year-old science teacher from the Netherlands. “I started thinking about it in 2004, watching the last transit from a sports field and having trouble accurately timing the start and end,” he explains. A couple of years later, as smartphones became more and more popular, van Roode realised that they could provide the temporal and spatial precision he needed.

Now, through collaborations with a Dutch programmer and an American geographic information systems company, his app is being downloaded worldwide. “It makes us all eighteenth-century explorers,” he laughs. “A few taps of the screen, and you can find your precise time and place, and send in your results. And when you submit your own data, you’ll be able to see observations coming in from all over the world, along with people’s Twitter feeds and so forth.”

The transit, says van Roode, “is truly an event that unites people. They’re watching the same event together at precisely the same moment.”

With the event’s rarity and the long lag until the next one, both men agree there’s something special about being astronomically inclined in this particular window of time. (Though, both stress, viewers mustn’t literally watch the transit: “It’s dangerous to look at the Sun,” says Lomb. Find an observatory or local amateur group, he suggests, or project the transit on a wall through binoculars or a telescope.) While Lomb will enjoy the moment from Australia’s Siding Spring Observatory, van Roode and his smartphone will follow in the footsteps of Father Hell, and travel to Vardø for the occasion.

“I hope I have coverage,” he says suddenly, a little alarmed. “You know, it’s so remote.”