Eclipse!

Tomorrow (May 15th), the Moon will enter the shadow of the Earth. First part of it, then all of it will turn a deep red-orange. Everyone in America will see it, from 10:00 P.M. to 1:00 A.M. Two weeks later, the Moon will move in front of the Sun for a solar eclipse.

The Moon will be too close to the Earth then for a total eclipse. Instead, observers will see a ring of light -- an annular eclipse -- around the Moon. Solar eclipses always precede or follow lunar eclipses by two weeks.

But why isn't there a solar and a lunar eclipse every month? When the Moon is full, the Earth is between the sun and the Moon. Then we would expect a lunar eclipse, as the Moon falls into the Earth's shadow. It would work just the same way with solar eclipses, at New Moon.

The answer is that the Moon's orbit around the Earth is slightly tilted compared with the Earth's orbit around the Sun. If the Moon orbited the Earth in the same plane that the Earthıs orbit around the Sun, there would be a solar and a lunar eclipse each month. Solar eclipses wouldn't be so rare. We wouldn't travel long distances to see them. But the Moon's orbit is tilted about five degrees away from the Earth's. Most of the time the eclipse-causing shadows point too high or too low. Only when there is near-perfect alignment is there an eclipse. As these bodies circle each other, there is also near-perfect alignment two weeks later, and there is an eclipse of the other type.

This perfect alignment happens twice a year, during what are called eclipse seasons. These are usually around May and again around November. The next solar eclipse is on November 23rd, but is visible only from Antarctica. If you like cold weather, start making your travel plans now!

(5/14/03)