In a changing world, the cosmos provides some certainty. And at precisely 8.16am on June 6 next year, the planet Venus will begin to pass in front of the sun, looking from Earth like a small, black beauty spot drifting slowly across the face of the glowing orb.
It will be the last opportunity in a lifetime to witness this rare celestial event. For it is also certain that another transit of Venus will not occur until December 2117. ''It's the last chance people will have to see it for 105 years and six months,'' says Dr Nick Lomb, a consultant astronomer for the Powerhouse Museum's Sydney Observatory.
Only seven transits, which come in pairs eight years apart, have occurred since the telescope was invented 400-odd years ago.
The first, in 1631, went unobserved, because it was night in Europe. The last, in 2004, was seen from space, observed with high resolution telescopes on Earth and photographed by countless people around the world.
The five transits in between played a major role in history, inspiring long, and at times dangerous, global scientific expeditions.
Without one such journey, Australia might not have become a British colony. Captain James Cook sailed to Tahiti to observe the 1769 transit, before heading south and mapping the east coast of the continent.
''In effect, modern Australia owes its existence to a celestial event,'' says Lomb, the author of a new book, Transit of Venus, 1631 to the present.
With the next one fast approaching, he tells the many tales of triumph, misadventure and even death for those who made witnessing a transit their scientific quest.
Apart from wars, extremes of temperature, and disease, their biggest enemy was undoubtedly the weather, as it will be next year.
Its impact is no better illustrated than by the travails of the French scientist Guillaume Le Gentil, whose transit trip was the longest in history. He set out to observe the 1761 transit in India, but learnt on arrival that the French colony had fallen to the British. He was still at sea, on his way back to Mauritius, when it occurred.
Rather than return to France, he waited for the 1769 transit, arriving back in India a year early, to be safe. But following two months of clear skies, the big day turned out to be overcast, Lomb said. ''He was in state of shock. And I don't blame him.''
As Le Gentil wrote: ''It seemed I had crossed such a great expanse of sea, exiling myself from my native land, only to be the spectator of a fatal cloud which came to place itself before the sun at the precise moment of my observations, to carry off from me the fruits of my pains and fatigues.''
Transits are also timely reminders of a changing world. William Harkness, a US astronomer who watched the 1874 transit from Hobart, noted that when the 1761 and 1769 transits occurred, ''the intellectual was awakening from the slumber of ages, and that wondrous scientific activity, which has led to our present advanced knowledge, was just beginning''. Looking ahead, he wrote: ''What will be the state of science when the next transit season [2004 and 2012] arrives, God only knows.''
Lomb says the 2012 transit will prompt questioning about life in 2117. ''When Harkness wrote those lines people were happily thinking about the progress of science. But these days there are so many unpleasant scenarios into the future.''
The German mathematician Johannes Kepler was first to predict a transit of Venus would occur, in 1631. Although this event was missed, a British man, Jeremiah Horrocks, realised there would be a second in 1639.
He and his friend William Crabtree were the only two known observers. It was ''a most agreeable spectacle. A spot of unusual magnitude and of a perfectly circular shape'', Horrocks said.
For the 1761 and 1769 transits, many astronomers journeyed to far-flung places to set up their observatories, with one Frenchman travelling more than 2000 kilometres by snow sled to observe from Siberia. Their main motive was that the transit provided a way to measure the distance between the Earth and the sun.
And the results of their observations led to a figure of 152.1 million kilometres, not too far off the present day value of 149.6 million kilometres.
Photography was widely used for observations of the 1874 and 1882 transits, including at the Sydney Observatory. And that is also where the crowds gathered for the 2004 transit, although one 17-year-old girl told the Herald: ''It's a pretty big turnout for a dot.''
Australia would be one of the best places in the world to see the 6½-hour long transit next year, Lomb said. In Sydney, a location overlooking the Pacific Ocean would be ''ideal''.
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