IN 1769 more than 100 scientists sailed to distant parts of the globe to watch the rare transit of Venus.
On Wednesday scientists and citizen scientists alike will have to travel no further than their local school, university or observatory to observe the once-in-a-lifetime event.
It's a rare alignment - happening just 25 times in the past 2000 years - which sees Venus line up between the Earth and sun. Back-lit by the sun, Venus will appear as a tiny black disc slowly moving across the sun's surface. It won't happen again until December 2117.
Eastern and central Australia will be one of the few places in the world to see Venus' entire six-hour journey across the sun, beginning just after 8am.
Melbourne Planetarium astronomer Dr Tanya Hill said Venus would first be seen at the bottom right of the sun at about 8.16am. It will be visible as a tiny black spot on the sun until 2.45pm, although Sydney Observatory astronomer Geoff Wyatt said the transit should only be viewed safely through a solar telescope, solar binoculars or solar glasses.
If there is cloud cover - and weather forceasts indicate there will be - the event can be followed online at www.transitofvenus.com.au
One of the earliest to document the transit of Venus was navigator and surveyor James Cook, whom the Royal Academy dispatched to Tahiti on the HMS Endeavour to observe and record the rare event in 1769.
It was hoped that the observations would help solve one of the great puzzles occupying 18th-century scientists - just how big was the solar system?
While we now know that the Earth is about 150 million kilometres from the sun and Venus about 108 million kilometres from the sun, for Cook's contemporaries it was a mystery.
It was Edmund Halley who in 1716 theorised that by observing the transit of Venus from different points of the globe, scientists could calculate the scale of the solar system. This sparked some of the first great global science expeditions, with scientists dispatched for the 1761 and 1769 transits.
Following Cook's observations in Tahiti, the Endeavour continued south to chart the east coast of Australia - confirming another prize puzzle of the era; that there was a southern continent.
ALMOST half of the moon will be blocked out by the moon’s shadow during a partial lunar eclipse tonight.
Melbourne Planetarium astronomer Tanya Hill said the lower part of the orb would disappear from about 8pm. The greatest eclipse will be at 9.03pm, and it will all be over by 10.06pm. ‘‘You don’t need any special equipment to look at it safely, which is the great thing about lunar eclipses,’’ Dr Hill said.