While most of the world gets an interplanetary show, Tucsonans will be left in the dark

The transit is coming! The transit is coming!

No, this is not another light-rail fantasy brought to you by the Birkenstock crowd. This is a transit of Venus across the face of the sun, a sort of micro-mini-eclipse that only happens four times (or less) every 243 years.

I've always been fascinated by astronomy. In fact, I briefly considered a career in the field until I soured on the experience after my astronomy professor in college kept hitting on my girlfriend. She kept telling me it was no big deal, but it creeped me out. The guy wore sandals, for crying out loud!

Anyway, on June 8, the orbits of Earth and Venus will have the planets lined up in such a way that most of the people on Earth will be able to see Venus cross in front of the sun. For a glorious six hours, Venus will appear as a black dot casually making its way across the face of the sun. People in Europe, Africa and Asia will be able to see most or all of the transit, while those in Australia and most of North and South America will be able to at least part of it. About the only people who won't see any of the transit are those unlucky souls in parts of Chile, Argentina, Mexico and the West Coast of the United States.

According to Sky & Telescope magazine (a sexy read, indeed), we in Tucson, alas, will not be able to see it. This is just bad luck, although I'm sure that the people on talk radio will find some way to blame it on Bill Clinton. However, it might be the perfect time to take that trip to Murmansk that you've been dreaming about. You can see the whole thing from there.

The reason this event is so rare is that the orbits of Venus and Earth aren't on the same plane. The orbit of Venus is inclined roughly 3.4 degrees to that of Earth, so even when the sun, Venus and Earth are lined up, Venus is either too high or too low (above or below our ecliptic) to cross the face of the sun. But every now and then, things line up just right, and the heavens put on a show.

What's even cooler is that even though it hasn't happened since 1882, after it happens in June, it will happen again in 2012. That's because Venus takes 224.7 days to orbit the sun, so 13 Venus years are almost exactly equal to eight Earth years, and in 2012, the two planets will be right back where they were, in relative terms.

But if you miss it both of these times, you'll have to wait until 2117 to see it again. I'm not sure that'll be worth the wait; I mean, who knows what the Bush family will have done to the economy by then? (I'm one of those Baby Boomers who, after waiting my entire life up to that point to see Halley's Comet in 1986, was so pissed off by the virtual no-show that I vowed to stay alive until 2061 just to see if it could do better the next time around.)

Since a transit of Venus is barely visible to the naked eye (and highly dangerous to attempt), mankind has only known about it for a few hundred years. It was first predicted by German astronomer Johannes Kepler in the 17th century. In fact, Kepler determined that there would be a transit of Mercury in November 1631, followed by a transit of Venus less than a month later. So excited was he by these discoveries that he up and died about a year before they happened.

There are at least three written accounts of the transit of Mercury in 1631, but the transit of Venus wasn't visible from Europe, and although Kepler had done his best to spread the word around the world, there are no accounts of anybody having seen it.

By the time it came back around in 1761 and 1769, British Astronomer Royal Edmond Halley (he of the sucko comet) had devised a mathematical formula to use the transit to determine, among other things, the distance from the Earth to the sun and the scale of the entire solar system. Halley's parallax method required observations of the transit from two or more points of widely varied latitudes.

And so it was that the first voyage of Captain Cook on the Endeavour was launched. Halley wanted Cook to observe the transit from the South Pacific so they could compare notes at a later time. Cook did so from a place in Tahiti that is still known to this day as Point Venus.

The top scientist in the American colonies at the time, a man named David Rittenhouse, looked through a telescope at the event and fainted dead away from the sight. And who could blame him?

I wonder if the Weekly would pay for my trip to Murmansk.

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