Tuesday, July 14, 2009

Chasing totality

It is one of the more interesting cosmic coincidences that the Sun and the Moon have almost exactly the same apparent size in the sky -- about half the width of your little finger held at arm's length. The Sun is 400 times bigger than the Moon; it is also 400 times further away.

This means that the tip of the Moon's shadow -- a long, thin wizard's cap of darkness -- just barely reaches to the Earth. In fact, since the distances to Sun and Moon are not constant, sometimes the Moon's shadow reaches the Earth's surface and sometimes it doesn't.

In January of this year, when the Moon slipped directly in front of the Sun, the tip of its shadow swept high above the Indian Ocean. If you had been in a boat somewhere beneath the shadow tip, you would have seen an annular eclipse of the Sun -- the Moon not quite covering the Sun's face. The Sun appeared as a thin ring of light, inspiring to look at, but not quite as spectacular as a total eclipse.

On July 22 the situation will be rather different. The Moon is about as close to the Earth as it ever gets -- about 56 Earth radii -- and as it passes in front of the Sun its shadow slices deep into the Earth, like the tip of a fencer's rapier into an opponent's cheek (ouch!). If you are in the path of the shadow, you will see a total solar eclipse, one of the longest ever -- depending on where you are, more than six minutes of darkness.

And where you should be is China, unless you have managed a boat in the western Pacific. The shadow will sweep across China, from southwest to northeast, including the city of Shanghai, population 14 million, the sixth largest city in the world. As Guy Ottewell has noted, more people will experience a total solar eclipse at one moment than ever before in history.

I made it to the Black Sea for the total solar eclipse of 1999, and to southern Turkey for the total solar eclipse of 2006, with cloudless skies both times. China seemed rather more dicey -- a long way to go for an overcast sky. I suppose the next total solar eclipse I will chase will be the one that sweeps across the United States in 2017, just missing my old home town of Chattanooga, assuming, of course, I haven't reached totality myself.