Venus Gets in Our Way for the Last Time
NASA's TRACE satellite captured this image of Venus crossing the face of the Sun as seen from the Earth's orbit. The last event occurred in 1882. The next Venus transit will be visible June 5, 2012. Photographer: NASA/SSPL/Getty Images
Unless you think you’ll live to see the year 2117, today is your last chance to witness one of astronomy’s great marvels: the transit of Venus. It’s a rare event, when the solar system’s second planet makes a rare passage directly between the Earth and the sun.
The transit can’t compete with the celestial fireworks of aurora borealis or a full solar eclipse. However, the transit has few parallels as a symbol for how far humans have come in our understanding of the Solar System and how collaboration stretching several generations can solve big problems.
The first recorded sighting of the passage was made in 1639. Less than 25 years later, the 17th-century mathematician Rev. James Gregory hypothesized that the passage of Venus, if viewed from multiple observation points, could be used to measure the distance between the Earth and Sun. Sir Edmund Halley, namesake of Halley’s Comet, hammered out the details of the technique in 1716. Unfortunately for him, it couldn’t be tested until Venus made its next pass, still another 45 years later.
Sufficiently precise measurement eluded 176 scientists, stationed in 117 spots around the world, in 1716. More than $1 million – a staggering sum at the time -- was spent internationally in 1874 to take hundreds of photographs. It still wasn’t enough.
The effort had caught public attention by 1882, and amateur stargazers took to the streets to view the historic passage. Scientists took thousands of photographs. After another ten years of analysis, Simon Newcomb, head of the U.S. Venus Transit Commission, deduced a distance between Earth and Sun of about 92,702,000 miles. The actual distance now recognized, called an Astronomical Unit, is 92,955,807.
Who knows what questions science will bring to the 2117 Venus transit. The sentiment behind an 1882 statement by William Harkness, director of the U.S. Naval Observatory, still rings true: "What will be the state of science when the next transit season arrives God only knows. Not even our children's children will live to take part in the astronomy of that day. As for ourselves, we have to do with the present."
In the next 12 hours, millions of people around the world will witness the transit of Venus (see instructions on how to do it without causing damage to your eyes).
The striking thing about the study of the transit of Venus was the willingness of scientists to embark on a project that wouldn’t reach fruition until after their lifetimes. The great scientific challenge of the 19th century was to measure and catalogue and begin to understand the natural world. The great scientific challenge of this one is to assure its survival. Solving that problem may require the same degree of cooperation.
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