Skywatchers on seven continents, including Antarctica, will be able to see all or part of the Venus transit:
- Should only be observed with telescopes outfitted with solar filters to protect the eyes
- Internet will be a hub of activity, with live video and pictures from an armada of space- and ground-based observatories
- Even astronauts aboard the International Space Station are joining in the event
Space station flight engineer Don Pettit said: "I've been planning this for a while," in a NASA interview. "I knew the transit of Venus would occur during my rotation, so I brought a solar filter with me."
One of the rarest astronomical events will take place in the early hours of Wednesday morning when Venus passes directly between the sun and Earth. It won't happen again until 2117.
Stargazers are being warned not to look directly at the sun and to use special glasses to avoid damaging eyesight.
During the astronomical fly-by Venus appears as a small, dark round spot.
It is due happen at around 5am UK time.
There are more details about what to expect on NASA's website.