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Venus transits the Sun. For a few hours on June 5, 2012, Venus will appear as a dark, perfectly round disk in silhouette against the Sun. Contacts are the times at which the edges of the Sun and Venus just touch.

A rare event...

Venus is the second planet from the Sun and its orbit is, therefore, smaller than Earth's: While our planet circles the Sun in one year, Venus completes its orbit in only 225 days.

Every 584 days, Venus catches up to our planet and passes between the Earth and Sun: This is known as inferior conjunction. Normally though, Venus’ orbit, which is tilted 3.4° with respect to that of Earth, carries it either above or below the Sun’s disk. But on rare occasions, when the inferior conjunction occurs in early June or early December, Venus’ alignment carries it directly in front of the Sun. This is called a transit.

Transits of Venus usually occur in pairs, eight years apart: Each pair is separated from the next by either 105 ½ or 121 ½ years, in alternating sequence. The June, 2012 transit is the second of such a pair — in fact, you may have seen the 2004 transit of Venus. But who can claim today with absolute certainty that they'll be around to witness the next transit, in December, 2117?

...but very noteworthy

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.