Sunday, January 25, 2009

Chasing eclipses

Every spherical object in the solar system has a conical shadow pointing away from the Sun. Even the head of a spacewalking astronaut wears a wizard's cap of darkness one hundred feet long. The Moon's shadow cap, by wonderful coincidence, is almost exactly as long as the average distance of the Moon from the Earth. During a total eclipse of the Sun, the Moon's shadow scratches the Earth's surface like the tip of a rapier. To see a total solar eclipse, one must be standing exactly where the shadow's tip intersects the Earth.

Tomorrow, that will almost happen in the southern Indian Ocean. The Earth, Moon and Sun are lined up, but the Moon is at apogee -- its furthest distance from the Earth -- and the tip of the shadow does not quite reach the surface of the Earth. If you managed to make your way by boat to the eclipse path you'd see the Sun not quite fully covered by the Moon. A ring of light! An annular eclipse. Just at the end of the eclipse, the path of annularity reaches Indonesia. Perhaps Mark will see the ring. At the least he will see a partial eclipse of the Sun, clouds permitting.

All of which reminds me that on July 22 of this year I won't be in China for what will be the longest total solar eclipse between 1991 and 2132. When the alignment of astronomical bodies occurs, the Moon will be at perigee -- closest to the Earth -- and the shadow's rapier tip will slice deeply into the Earth. I was on a ship in the Black Sea for the total solar eclipse of August 11, 1999, and in southern Turkey for the total solar eclipse of March 29, 2006. But China defeated me. I will console myself in late April with a week-long walk along the Ridgeway in England with sons Tom and Dan.

Now, if I can hang on for another eight years, I will certainly be somewhere along the track of the eclipse that slices across the United States in 2017.

Why not just wait until a total solar eclipse comes to you? You might wait a very long time. Take a 12-inch diameter terrestrial globe such as you might have in your home or schoolroom, and every year or so draw a random line 10 or 12 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 the paths of total solar eclipses. How long until the entire globe is painted black with shadows? That is: What is the longest time that any place on the Earth's surface would have to wait for a total solar eclipse? Mathematical astronomer Jean Meeus has done the calculation, and the answer turns out to be 4500 years.