Thousands watch rare transit of Venus across the sun

Crowds of stargazers turned out to see Venus transit across the sun in an event that will not be repeated until 2117.

Venus passes across the sun

A helicopter flies past as the planet Venus makes its transit across the sun over Tijuana, Mexico

A bird sits on top of one of the domes of the Taj Mahal in Agra, India, as Venus passes in front of the sun 

It began just after 10pm GMT over the Americas, and was magnified as a small black dot on the surface of the sun.

The planet's transit took almost seven hours and was watched by hundreds of thousands on live broadcasts online.

One of the best places to see the phenomenon, which happens in a 100-year cycle, with two transits eight years apart, was Australia.

A sell-out audience of 1,500 people watched from the Sydney Observatory in the country that has historic links to the astronomical event.

British explorer Captain James Cook charted the movement of Venus over the sun ahead of his discovery of Australia.

Captain Cook set sail for Tahiti on HMS Endeavour to record the transit that occurred in 1769, and after a successful observation he was sent to seek the "great south land" thought to exist in the Pacific Ocean.

During the voyage, he charted the east coast of Australia, staking a British claim in 1770.

"It's not like an eclipse where you've got something blotting out the sun," Fred Watson, astronomer-in-chief at the Australian Astronomical Observatory, said.

"Venus is 100th of the diameter of the sun so it's essentially just a black spot superimposed on the disc of the sun, but it moves across from one side to the other."

The transit of Venus allows scientists the chance to learn more about the atmospheres of planets as they pass in front of stars.

Between 2000 BC and 2004, Venus has passed the sun 53 times but has only been observed six times – in 1639, 1761, 1769, 1874, 1882 and 2004 – because of the magnification needed to see it properly.

Richard Harrison, co-investigator at Nasa's Solar Dynamics Observatory, said: "A transit is a wonderful and rare sight; when you consider the vastness of the sky, for a planet to pass the disc of the sun is pretty unusual – and you do have to wait until 2117 for the next one."