Friday, March 24, 2006

TSE2

On the ground in Istanbul, and yes the hotel (on a narrow little street in the old city) has free WiFi. A couple of days here seeing the sights, then to the south coast for the eclipse on Wednesday.

Most people know why a solar eclipse happens: The Moon blocks the Sun's light. What is less well understood is why total solar eclipses are so rare. Think of the Earth as a grapefruit. On this scale, the Moon would be a grape about 20 feet away. Both Earth and Moon cast conical shadows pointing away from the Sun (Shelley's "pyramid of night/ which points into the heaven."). To have a total eclipse of the Sun, the Moon's conical shadow must touch some part of the Earth's surface.

And here's the kicker: The Moon's shadow is almost exactly as long as the average distance of the Moon from the Earth (think of the grape with its 20-foot-long shadow tapering to a point near the grapefruit Earth). Because of the tilt of the Moon's orbit to the plane of the ecliptic, most months the tip of the Moon's shadow passes above or below the Earth, but about twice a year, when the Moon is near the ecliptic plane (the plane of the Earth's orbit), a solar eclipse is possible.

But it's not a sure thing. The Moon moves around the Earth in an almost perfectly circular orbit, but the Earth is not quite at the center of the circle. Sometimes the Moon is a bit further from the Earth, and sometimes a bit closer. When the Moon is near apogee -- its greatest distance from the Earth -- the tip of its shadow does not quite reach to the Earth's surface and a total solar eclipse cannot occur. When the Moon is at perigee -- its nearest distance to Earth -- the rapierlike tip of its shadow just reaches Earth, or even extends a bit beyond.

To see a total solar eclipse, one must be in the narrow path where the rapier's tip slices the surface of the Earth -- like a fencer's rapier scoring the cheek of his opponent.