This Tuesday or Wednesday, depending on where you live, sky-watchers around the world will be able to see a cosmic spectacle known as a transit of Venus. The events are so rare that only six Venus transits have been observed since the invention of the telescope more than 400 years ago.

Transits happen when a planet crosses between Earth and the sun. Only Mercury and Venus, which are closer to the sun than Earth, can undergo this unusual alignment.

The last Venus transit was in 2004—above, the planet glides across the rising sun in a picture taken during the event from the North Carolina coastline. After 2012, we won't see another transit of Venus until 2117.

"People watching this event through some form of safe solar viewer will see the small, dark silhouette of Venus crossing the sun's face over the course of about six hours," said Jay Pasachoff, an astronomer at Williams College in Massachusetts. (Read a Q&A with Pasachoff about Venus transits.)

"The effect won't be visually impressive, but that black dot against the sun is a remarkable thing to see."

Where can I see it?

The whole transit is visible from Alaska, parts of northern Canada, and from New Zealand, much of Australia, Asia and Russia. In the US, the transit will be in progress as the sun sets on 5 June. In East Africa, Europe and Scandinavia, the transit will be under way as the sun rises on 6 June. Much of South America and western Africa will not see the event.

How long does the transit last?

Venus takes nearly seven hours to cross the face of the sun, but the event is divided into four "contacts" that mark different phases of the transit. Venus makes first contact when it encroaches onto the disc of the sun. Twenty minutes later, on second contact, the planet will be fully silhouetted. On third contact, at 5.37am BST, Venus will begin to leave the sun, and the transit will be over on fourth contact at 5.55am BST.
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