For just over six hours on Tuesday, skygazers enjoyed a rare celestial sight - the passage of Venus in front of the Sun.
No-one alive today had witnessed such a transit before, as the last event occurred in 1882. Venus's orbital plane, slighted tilted to Earth's, will produce a repeat performance in 2012. But the next after that will be in 2117.
The phenomenon was visible to an estimated five billion of the planet's six billion people. Europe and Africa saw the entire transit, as was most of Asia. East Asia and Australia missed the end and the Americas missed the beginning.
But while many enjoyed clear views, through filters or projectors, some were disappointed. "We could barely see the Sun through breaks in the clouds," said Maki Yanagisawa, an official at the Japanese Youth Science Museum on Hokkaido.
Observations of six previous transits have been recorded, with the earliest being in 1631. Astronomers have exploited the events to make many significant breakthroughs, such as calculating the distance to the Sun and, in 1761, to discover that Venus has an atmosphere.
Astronomers today are also using the transit to further their research. A number are using the passage to test the techniques used to spot and analyse planets orbiting other stars (New Scientist, 6 June). Such observations could reveal the direction a planet is rotating in and whether it has an atmosphere or not.
Others hope to discover more about the composition and winds in the upper atmosphere of Venus itself. This region has been poorly observed until now, so Timothy Brown, from the US National Center for Atmospheric Research in Boulder, Colorado, observed the transit using a 70-centimetre solar telescope in the Canary Islands.
In 2001, Brown found sodium in the atmosphere of a planet orbiting a star 150 light years from Earth and is applying the same spectroscopic techniques to Venus. He is examining which wavelengths of the Sun's light are absorbed by the atmosphere between 65 and 85 kilometres above Venus's surface - above the planet's thick clouds. This will give the composition.
And analysing Doppler shifts in the light seen should reveal information about winds. "This is Venus's the first transit in front of the Sun since quantitative astronomical spectroscopy was invented," Brown says.
All comments should respect the New Scientist House Rules. If you think a particular comment breaks these rules then please use the "Report" link in that comment to report it to us.
If you are having a technical problem posting a comment, please contact technical support.
