The transit of Venus is when the planet Venus crosses between the sun and the Earth. It doesn't happen very often, and it has been an important event in times gone past.
JONATHAN HARLEY: Dr Nick Lomb has been watching the planets for decades.
Now he's winding up for one of the rarest heavenly events, seen only five times in recorded history.
DR NICK LOMB: I think it rates very much with big events like Haley's Comet back in 1986 and the total eclipse of the Sun.
It is an event that has not been seen by anybody alive.
So it is something very exciting to watch.
JONATHAN HARLEY: On Tuesday afternoon, a small black dot will appear to pass across the face of the Sun for the first time in 122 years.
Before the advent of more accurate technology, the transit of Venus held the key to calculating the Earth's distance from the Sun.
But the transit of Venus remains a major historical event - one with particular resonance for Australia.
JONATHAN HARLEY: The Transit of Venus in 1769 inspired the biggest international scientific effort the world had seen.
Scores of European observing teams were sent out around the globe, the most important of all, the British expedition to Tahiti.
DR NICK LOMB: Tahiti was just in the right spot in the Southern Hemisphere from which both the beginning and the end of the transit could be observed.
JONATHAN HARLEY: And leading the way was Captain Cook. Among those on board the 'Endeavour', astronomer Charles Green and the young, rich and passionate botanist Joseph Banks.
In Tahiti, having established Fort Venus, Cook and his colleagues recorded the cosmic event.
But the transit of Venus was just the start of Cook's mission.
The 'Endeavour' captain had been given sealed instructions not to be opened until after making his astronomical observations.
PAUL BRUNTON: He did his observation of the transit and then opened his secret instructions, and they were to sail south and to see whether the south land existed.
JONATHAN HARLEY: The rest, as they say, is history.
PAUL BRUNTON: Well, for Australia it was the crucial event in Australian history, because without the observation of the transit of Venus we may not have had Cook discovering the east coast of Australia.
JONATHAN HARLEY: And if you miss Tuesday's transit, or clouds block the view, don't panic.
The next one will be relatively soon - in 2012.
Though never will the transit of Venus hold such significance as its 18th century appearance, which changed the fate of the South Pacific.
Itís transit time!
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