Captain Cook had his eye on the cloud planet well before sighting our coastline.
SOME things are rare. A blue moon may occur once every two or three years; great comets are perhaps seen once in a generation; a premiership flag for the Bulldogs has not been seen since the 1950s. But all these are small potatoes compared with the transit of Venus. If you don't see the one on Wednesday, June 6, you'll have to wait another 105 years for the next.
You might wonder why so much is made of Venus lining up in an extremely straight line between us and the sun, so its sphere appears to move across the face of the solar disk - or transit - as a small, dark silhouette.
In 1663, Scottish mathematician James Gregory suggested using a transit of Mercury to discern the distance between the Earth and sun, but it was Edmond Halley some years later who proposed a transit of Venus would be more accurate. The process would require great precision in timing of the event and measurement of the planet's path across the disc of the sun.
Furthermore, it could only be successful if observations were conducted from many positions on Earth, but by the end of it, via Kepler's laws of planetary motion, we would know the size of the solar system - one of the great unanswered questions for early astronomers.
This noble quest was pursued through one of the earliest examples of international scientific co-operation. A number of nations in concert sent expeditions to many parts of the world in 1761 and 1769 to observe the phenomenon. Transits of Venus occur in pairs, eight years apart, but then do not recur for more than 100 years.
The 1769 transit has a strong connection with Australia, as James Cook's voyage in the Endeavour was first destined for Tahiti to observe the transit, and second to continue on to map the eastern coast of the ''Great Southern Land''.
Melbourne Observatory has a direct link with the transit of 1774 and two of the instruments installed to view the transit at that time are still in operation. They were used for the most recent transit, in 2004, and will again be used to see the final transit of Venus to be observed by anyone alive today.
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