Transit of Venus: Early 'tape measure' for size of our solar system (+video)

The next transit of Venus occurs June 5. Astronomers once used the transit of Venus across the sun to come up with the 'astronomical unit' ? the distance from Earth to our sun.?

By Joe Rao,?Space.com / May 30, 2012

The planet Venus, black spot, crossing the sun is photographed through a telescope in Hove, Belgium, in this June 8, 2004 file photo. On June 5, 2012, Venus will pass across the face of the sun, producing a silhouette that no one alive today will likely see again. This June's transit, the bookend of a 2004-2012 pair, won't be repeated until the year 2117.

(AP Photo/Geert Vanden Wijngaert, File)

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On June 5, the planet Venus will appear to cross in front of the disk of the sun, appearing as a small black dot. The historic skywatching event, called a transit of Venus by astronomers, is among the rarest of predictable phenomena and one that has amazed scientists and observers throughout history.

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Venus transits occur like clockwork, appearing in pairs eight years apart, either in June or December. On this scheduled, a pair of December transits follows a June pair after 105 1/2 years. A June pair will come 121 1/2 years after a December pair. As seen from Earth, only transits of Mercury and Venus are possible.

Transits of Venus and Mercury hold an interesting place in astronomical history. The main reason is the slightly different times that the events occur as seen from different locations on the surface of the Earth. The diameter of our planet is appreciable when compared to the distance to Mercury and Venus, allowing astronomers to triangulate on the planets from various points on Earth when they are seen against a bright background as the surface of the sun.

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In this way the difference between the distances of the planet and the sun can be found. Because of its considerably larger size, Venus is better for this purpose, and in fact, during the 18th and 19th centuries those rare transits of Venus provided astronomers with the best opportunity for measuring the absolute scale of the solar system from Kepler's third law. Such a value was poorly known at that time. Edmund Halley ? he of the comet fame ? was the first to realize that transits could be used to measure the Earth's distance from the sun, a distance called the "astronomical unit." [Gallery: Transits of Venus Throughout History]

Today, this method has been superseded, chiefly because those moments when either Mercury or Venus begins moving onto or off of the sun require precise accuracy, which more than two centuries ago was always difficult to attain.

Since 1882, transit observations ? and all the old methods of determining the astronomical unit ? have been hopelessly outmoded by modern techniques. Tracking of interplanetary spacecraft, and especially radar ranging to the inner planets, finally achieved the precision sought by astronomers for so long. Indeed, the upcoming Venus transit will be looked upon as merely a chance to view a rare and striking event.

Nonetheless, elaborate expeditions that were mounted to observe the transits of Venus in 1761 and 1769 provided astronomers with their first good value for the distance of the Earth to the sun (called an astronomical unit).

Trials and tribulations

Kepler had predicted that a transit of Venus would take place on Dec. 6, 1631, but, unfortunately, a large wind and rainstorm obscured the sun across most of Europe, preventing any observations.

An obscure young English astronomer, Jeremiah Horrocks, scrutinized Kepler's tables and came to the conclusion that another transit of Venus was to occur on Dec. 4,1639. Horrocks was ready for the big event, carefully examining the sun even the day before ? just in case he erred in his calculations. On Dec. 4 (a Sunday), under scattered-to-broken cloud cover, Horrocks was at his telescope from sunrise until 1 o'clock, at which point he interrupted his vigil by "business of the highest importance" ? conducting divine services at his church.

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