The transit begins at 11.09pm (BST) and lasts for six hours and 40 minutes. Times can vary by seven minutes depending on the location of the observer.
Skywatchers on seven continents can see the Venus transit, which should only be observed with telescopes fitted with solar filters to protect the eyes.
The skies over Britain will be disappointingly cloudy and some observers say parts of Australia will get the best views of the spectacle.
The 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.
"I've been planning this for a while," space station flight engineer Don Pettit said in a Nasa interview.
"I knew the transit of Venus would occur during my rotation, so I brought a solar filter with me."
Sky