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Transit of Venus

One June 5, 2012, U.S. national parks were able to see a rare event, the Transit of Venus. In the hours before sunset, every park in the contiguous United States, Hawaii, and the Virgin Islands was able to view most of the transit as Venus moved across the face of the sun..

Transit of Venus
2012 Transit of Venus—June 5, 2012

The transit of Venus is a special alignment of the Sun, the planet Venus, and the Earth. Due to this Solar System geometry, Venus transits usually occur in pairs, eight years apart, about once a century. In the Pacific, on June 5, 2012, for over six hours, Venus' tiny silhouette moved across the Sun's disk. On the scale of a human life, it is a rare astronomical event. It is so rare that the 2012 transit will be only the 54th occurrence since 2000 B.C.E. The 55th transit will be in 2117. Since Mercury and Venus are the only two planets that orbit between the Sun and the Earth, these are the only planets in our solar system in which this amazing phenomenon can be observed.

World Explorers Breaking the Frontiers of Modern Science

Jeremiah Horrocks
A 1903 portrait depiction of Jeremiah Horrocks, show projecting the Sun through a small telescope, observing his predicted transit of Venus in 1639.

For centuries the second planet in our Solar System, Venus, held the key for estimating the size of the solar system. Nearly 300 years ago, astronomers theorized that observations of the silhouette of Venus crossing the front of the Sun, combined with the mathematics of geometry, can be used to calculate the scale of the solar system. Thus began a scientific endeavor to observe and time this rare event. It would take centuries before explorers and astronomers would capture enough observations to realize how big our corner of the Universe is.

We do not know for certain if the transits of the inner planets were observed by ancient cultures. Before the invention of the telescope, Venus, to many appeared as a bright point of light. Before solar filters, observing the Sun had horrible blinding results, resulting in various projection methods to observe the Sun's disk. In 1610 Galileo Galilee was the first to observe Venus through a telescope and see it as a disk, not a point of light. In 1627 Johannes Kepler (using the detailed data of Tycho Brahe) discovered the three laws of planetary motion and made the first Venus transit predictions. It was this work that inspired Jeremiah Horrocks to more accurately calculate and predict the eight year interval between the paired Venus transits and successfully record the details on December 4, 1639. The 1639 transit of Venus observations inspired mathematician and inventor of the reflecting telescope, James Gregory, to theorize how to calculate the size of the solar system by measuring the transit event. It was during the next transit of Venus on June 5, 1761, that a Russian astronomer, Mikhail Lomonosov, eager to test these new theories, witnessed a "halo" around Venus as it reached the dark edge of the Sun, indicating that Venus has an atmosphere.

Eight years later, an exploration race developed amongst scientists to measure the next transit of Venus event. In 1769 many European expeditions to the South Pacific were launched to observe the transit of Venus and calculate the size of the solar system. The most famous of these expedition ships was the Endeavour, which Captain James Cook sailed to the island of Tahiti to observe the Venus transit. It was after the transit that the Endeavour happened upon New Zealand and Australia's Great Barrier Reef. With observation such as these, humanity came to realize that the Solar System was immense. The Sun was over 93 million miles away from the Earth, meaning the Sun was very large; even at the speed of light it takes over 8 minutes for sunlight to reach the Earth.

Just as the Venus transits were once used by early astronomers to calculate the size of our solar system, modern astronomers make new discoveries today by observing transits of extrasolar planets orbiting around distant stars. When a planet transits in front of its parent star, it blocks a tiny fraction of that star's light. The tiny change in brightness is measured by the NASA Kepler Mission's space telescope as it scans one area of our neighborhood in the Milky Way. Modern Earth-based telescopes confirm Kepler's exoplanets candidates, and a new solar system is discovered.

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Last Updated: June 11, 2012