Saturday, August 11, 2007

Painting the planet in darkness


A week or so ago I posted a map showing the track of the solar eclipse that coincided with the Siege of Delhi in 1857. Here is another map from the same NASA atlas of eclipses, showing (in blue) all of the total solar eclipses between 2001 and 2020 (click to enlarge). I was on the southern coast of Turkey for the eclipse of March 29, 2006. I hope to be in China for the July 2009 eclipse. And if I can hang on for another decade, I will certainly be somewhere along the track of the eclipse that slices across the United States in 2017.

The Moon's conical shadow is as long and thin as a rapier, and by coincidence it is just about as long as the slightly variable distance between the Moon and Earth. Sometimes when the Moon gets between the Sun and Earth the tip of the shadow doesn't quite reach to the surface of the Earth and we have an annular eclipse (a thin ring of sunlight surrounds the Moon); these are the red bands on the map. But the really spectacular events are totality, when the tip of the shadow-rapier slices into the body of the Earth. The "gash" is the path of totality, the blue bands on the map. If you want to see a total solar eclipse you must be standing somewhere in this band, preferably near the centerline.

If the Moon were a bit smaller or a bit farther away, we wouldn't have total solar eclipses at all. If the Moon were bigger or closer, eclipses would be more common. As it is, the sizes and distances are such the these mind-blowing events are deliciously rare. As you look at the 20-year map, you can guess that the chance of having a total eclipse at the place where you live during your lifetime is small.

How long would it take for the entire map of the Earth to be "painted blue"? That is, what is the longest time any particular place on Earth would have to wait for a total solar eclipse? The answer: 4,500 years. Unless you want to travel, don't hold your breath. For more on this sort of thing, see my An Intimate Look At the Night Sky.