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June 8, 2004: Venus Transits the Sun

On June 8, 2004, the world will witness the first Venus' transit of the Sun since December 6, 1882. Nobody living today has ever seen Venus transiting (or orbiting in front of) the Sun. If we miss this Venus transit, we'll have another chance in eight years on June 6, 2021. (Venus, by the way, is famous for its eight year cycles.) After that, however, the next one won't be till December 11, 2117. Although Venus passes between the Earth and the Sun five times every eight years, the planet almost always orbits a bit above or below the Sun's disk.

Unfortunately for most of the Americas, we're not favorably positioned to watch this rare astronomical treat. In fact, at my home in Northern New York, we will -- if the skies are clear -- see only the last two hours of the six-hour drama, starting at sunrise -- 5:14 a.m. at Potsdam, NY. The western section of the United States won't get to see the transit at all. But should location or weather be a factor, you can always watch the historic event live.

Before you're too tempted to peer at Venus' heavenly silhouette on your own, be forewarned that looking at the Sun can be very damaging to the eyes. If weather permits and you're in the Potsdam, NY area, you're invited to view the transit from the Clarkson University Observatory at the Potsdam airport. The observatory will probably be open around midnight, but call 268-6677 to make sure someone is there. Professor Jan Wojcik, the overseer of the observatory, has told me that the observatory is also open to the public on clear Friday nights.

Historically, transits of Venus were of paramount scientific interest, because it enabled astronomers to determine what's called the Astronomical Unit (AU) -- the mean distance between the Earth and the Sun. Even in Copernicus' day, educated people had a very good idea of the relative distances to the known planets in terms of the Astronical Unit -- the only problem was that no one knew how far that was. Click here to appreciate how clever astronmers in olden times discovered the relative distances to the planets.

In 1672, the astronomers Gian Cassini and Jean Richer used the parallax of Mars to estimate the AU at 87 million miles. (Click here, if you'd like a few more details.) That's not bad, considering that Sky & Telescope magazine lists the modern figure at 149,597,870 kilometers or 92,955,807 miles. Still, astronomers back then were distrustful of the result, and waited anxiously for a more trustworthy Venus' transit.

For the benefit of Canton and Potsdam viewers in northern New York, Professor O'Donoghue has sent me a map of the Venus transit, though she has made it abundantly clear that all credit is to be given to Fred Espenak. In fact, his web page is a gold mine of information, so you're encouraged to click here to further investigate the historic transit of Venus.