Saturday, March 25, 2006


The long, skinny conical shadow of the Moon just barely reaches the Earth. That's the little black dot in the graphic for yesterday's post. If the Moon were a bit smaller we wouldn't have total solar eclipses at all. And if the Moon were bigger the intersection of the shadow with Earth would be larger and eclipses wouldn't be so rare. By celestial coincidence, the relative sizes and distances of the Sun and Moon are such that we are graced with an extraordinary event that is deliciously rare.

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. Hang on for that long and the little black dot is certain to sweep across you no matter where you live.

A day on the town in Istanbul, visiting some places we missed last time.