Venus transits across the sun: Top 10 facts

A rare celestial phenomenon, it is visible when the Venus passes between the Sun and the Earth. None of us will likely see Venus pass the sun again, considering the next transit won't be for another 105 years.

The transit began shortly after 2200 GMT on Tuesday in parts of North America, Central America and the northern part of South America, and was visible, with magnification, as a small black dot on the solar surface.

All of the transit will be visible in East Asia and the Western Pacific. Europe, the Middle East and South Asia will get to see the end stages as they go into sunrise Wednesday, while North America saw its opening stage.

Venus, which is extremely hot, is one of Earth's two neighbours and is so close in size to our planet that scientists at times call them near-twins.

The passage between the Earth and the sun of the solar system's second planet should only be viewed through approved solar filters to avoid the risk of blindness, experts have warned.

Venus transiting the sun occurs in intervals of eight, 121, eight and 105 years. Today's transit is the last of this century.

Only six transits have ever been observed -- in 1639, 1761, 1769, 1874, 1882 and 2004 -- because they need magnification to be seen properly, though the event has happened 53 times between 2000 BC and 2004.

Scientists say the transit allows them to learn more about how to decipher the atmospheres of planets outside our solar system as they cross in front of their own stars.

The European Space Agency's Venus Express is the only spacecraft orbiting the hot planet at present and will be using light from the sun to study Venus's atmosphere.

And the NASA Hubble Space Telescope, which cannot view the sun directly, will use the Moon as a mirror to capture reflected sunlight and learn more about Venus's atmosphere.