Tuesday, March 08, 2005

Chasing darkness

Like Earth, the Moon wears a long conical shadow of darkness. By cosmic coincidence, the Moon's shadow is almost exactly as long as the distance from the Moon to the Earth. Roughly once every couple of years, when everything is lined up just right, the tip of the Moon's shadow brushes across the face of Earth, like the tip of a feather across your cheek or brow.

If you are in the path of the shadow, you will experience one of the most spectacular phenomena of nature, a total eclipse of the Sun.

But to see a total solar eclipse, you will almost certainly have to travel. Take a 12-inch diameter terrestrial globe such as you might have in your home or schoolroom, and every couple of years draw a random arc 10 inches long across its face with a black felt-tip marker. The line can be anywhere from North Pole to South Pole and in any hemisphere. These marks are typical of total solar eclipse tracks. Mathematical astronomer Jean Meeus has calculated that it takes 4500 years for the planet to be completely colored black. The chances are small that in your lifetime an eclipse track will brush your place of residence.

This year has no total solar eclipses. Twice, on April 8 and October 13, the shadow comes sweeping by, but the "tip of the feather" barely misses touching the Earth's surface, so a ring of sunlight remains -- an annular eclipse. The next total solar eclipse is March 29, 2006. More tomorrow.