On 6 June, an event that takes place only four times every two centuries will enthral the world's astronomers, as it has ever since the 1600s – but now it can provide priceless data in the hunt for habitable planets in deep space
A tiny speck will appear on one side of the Sun in a few weeks and slowly traverse the solar disc for a few hours. The movement of that little black dot may seem insignificant. But it is one of the rarest sights in astronomy, an event known as a transit of Venus. Miss this one and you will have to wait until 2117 for the next.
Earth's closest planetary neighbour, which is currently in close and spectacular alignment with Jupiter in the night sky, will make its passage across the Sun's disc on 6 June and can expect to make scientific headlines – for astronomers hope studies of the transit will provide them with key data for studying worlds that orbit distant stars.
"This transit is special because it is the last time in our lifetimes that we will have an opportunity to collect data for a planet as well characterised as Venus," said David Crisp of Nasa's Jet Propulsion Laboratory in Pasadena, California. "We will have to make the most of it."
Venus, for all its glittering beauty in the night sky and its association with the Roman goddess of love, is a deeply unpleasant world. It has a surface temperature of 460C, its dense atmosphere of carbon dioxide has incinerated or crushed all robot spacecraft that have landed on it and its surface is shrouded by thick clouds of sulphuric acid. Once thought to be a sister world to Earth, because of their similar sizes and orbits round the Sun, Venus is more like a vision of hell.
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