Wednesday, November 14, 2012

Darkness at noon

Report from Australia: The gals saw some of the partial phases, but totality was obscured by cloud. Sigh!

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 Monday's 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. With a clear sky.

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 that 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. An average wait is about 400 years.

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. Some folks now alive in Arkansas (2024/2045) or southern Illinois (2017/2024) will experience two total solar eclipses in their lifetime without leaving home.
But you can see from Monday's map how rare it is that two blue bands cross. Has any spot on Earth experienced three total solar eclipses in a lifetime? I would guess so, but I don't have the patience to churn through the NASA atlas.

I do notice, however, that our place in Exuma in almost dead centerline for the eclipse of 2045. Oh how I'd like to be sitting on my own terrace (the black dot) and watch that event. Six minutes of totality! Compared to two minutes for the Cairns eclipse and not far off the maximum. Alas, I'll be long gone. Maybe the house will still be in the family and aged children (Tom, our youngest, will be 75) or grandchildren will be there.
If rising seas haven't washed the house away.