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Historical importance of the phenomenon

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Venus approaches the Sun
Photo: NASA/SDO

A very noteworthy phenomenon

It was Johannes Kepler who computed the first prediction of a transit of Venus. The transit of 1631 was not visible from Europe and likely went unobserved. Curiously, Kepler's calculations were still rough and failed to predict a transit in 1639. Acting on his intuition, however, Englishman Jeremiah Horrocks tackled the task of recomputing the event: Completed with less than a month to spare, his calculations showed that Venus would indeed transit the Sun on December 4, 1639. In those days, the timeframe was too short to spread the word to other astronomers in Europe: Horrocks and his friend William Crabtree were, therefore, the only witnesses of the first transit of Venus ever observed.

In 1677, astronomer Edmund Halley – famous for proving that some comets are periodic, and accurately predicting the return of one of them – showed that transits of Venus offered a unique opportunity to calculate the absolute distance between the Earth and the Sun. To accomplish this feat, one would need to combine accurate timings of the ingress and egress of Venus in front of the Sun, observed from locations as widely separated as possible. Once the Earth-Sun distance was established, it would then be possible to determine the distance of all the planets in the solar system – and even the closest stars! At that time, transits were thus considered extremely valuable, scientifically speaking.

During the 18th and 19th centuries, a number of expeditions were mounted to observe transits of Venus (in 1761, 1769, 1874, and 1882) from the four corners of the globe. The goal was to accurately time the moments when the small, round silhouette of Venus entered and left the face of the Sun. For a variety of reasons, however, measuring the contact times with the required accuracy turned out to be a near impossible task. Overcoming these disappointments, 18th and 19th century astronomers were nevertheless able to establish the distance scale of our solar system.

Today, the distance to the planets is measured with great precision thanks to radar. Transits of Venus are of interest because they are rare, and because of the history they evoke. As well, they allow astronomers to refine techniques used to measure the transit of extrasolar planets as they pass in front of their stars. A phenomenon once reserved for scientists, the 2004 transit was the first one to be widely observed by the general public.

Transits of Venus
...
1631 December 7
1639 December 4
1761 June 6
1769 June 3
1874 December 9
1882 December 6
2004 June 8
2012 June 5-6 *
2117 December 11
2125 December 8
...

* The actual calendar date on which the phenomenon occurs depends on the time zone of the observation site.

What about Mercury?...

Closer to the Sun, Mercury also undergoes transits, and more frequently than Venus – as often as 13 times per century! However, Mercury is much smaller than Venus and farther away from us. The planet's tiny disk is therefore unsuited to reliably measure the Earth-Sun distance...

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