A rare alignment between Earth, Venus and the Sun allowed astronomers of the 17th and 18th century to calculate for the first time the precise distance of the Earth from the Sun. Now, astronomers around the world are preparing for the next transit of Venus that will take place on 5 and 6 June, 2012, a phenomenon that will not be repeated for the next 105 years.
On the afternoon of June 5, Venus will begin to pass between Earth and Sun, so it will become visible as a dark spot moving across the fiery solar disk.
“For centuries, the Venus has been considered one of the highlights for astronomers,” said Claude Catala, head of the Paris Observatory.
On the afternoon of June 5, the first phase of the transit will be observable in North America, Central America and the northern part of South America.
In Europe, Middle East and South Asia it will be possible to see only the final phase of the phenomenon just before the sunset of June 6.
The only areas from which the transit will be visible for its entire duration are East Asia and Western Pacific.
Only six transits of Venus have been recorded since the phenomenon was theoretically predicted in the 17th century.
Scientists soon realized that they could theoretically calculate the trigonometric Earth-Sun distance, called the Astronomical Unit (AU), by observing the same transit of Venus from different parts of the world.
For this reason, Britain and France sent their missions to distant places to record the passages of the 1761 and 1769.
The British mission in 1761 departed from Plymouth, but admitted assault by French ships and its members expressed their desire to go back.
Equally dramatic, but with more positive result was the British mission in 1769, when the great explorer Captain James Cook was sent to Tahiti to record the rare phenomenon.