IT'S A twice-in-a-lifetime moment.
On Wednesday morning, June 6, an event that takes place only four times every two centuries will enthral the world's astronomers, as it has since the 1600s.
People all over the world will be craning their necks to get a look at one of the rarest sky events possible, the Transit of Venus - and nobody alive will ever see this again. The next one will be in 2117.
Well-known Australian astronomer and writer for Australian Science magazine Dave Reneke said Australia, New Zealand, East Asia and the western Pacific are the best locations for viewing all of the 2012 transit, which starts soon after sunrise and finishes in the afternoon.
People in the United States can watch for it on Tuesday afternoon, June 5. Most other parts of the world will see some of the transit, with the exception of West Africa and most of South America.
"A Transit of Venus is when the small disc of Venus passes in front of the sun. The event takes several hours and has only been viewed six times in recorded history," Mr Reneke said.
"The last was in 2004 and was watched by millions who used telescopes to project images of the sun's disc and the dot of Venus on to cards or electronic monitors."
Mr Reneke also said it was the reason Lieutenant James Cook came to the southern hemisphere to view the transit of 1769.
"By comparing northern observations astronomers of those times were able to calculate the distance between the sun, Earth, and Venus," he said.
"It was pretty accurate; then he went on to discover Australia."
Scientists hope the 2012 transit will give them more clues about the atmosphere of Venus, plus provide priceless data in the hunt for habitable planets in deep space using similar transit methods.
You can view the sun directly by using specially made filters. Do not use sunglasses, smoked glass, exposed film or CDs as filters.
While these objects may reduce the amount of visible sunlight they don't reduce the radiation. Eye damage could be the result.
For the east coast of Australia, Venus starts to pass across the bottom part of the sun at 8.16am with the whole event over by 2.44pm. For Adelaide it begins at 7.16am and in Perth the transit will have just started at sunrise.
Mr Reneke's website davidreneke.com has free fact sheets and other downloadable information on the history of the transit and how to watch it safely from any city in the world.
Viewed six times in recorded history
Last visit was 2004
Next visit 2117
2012 transit begins on the east coast of Australia at 8.16am and finishes at 2.44pm on Wednesday
Use special equipment to watch or photograph the event