Tuesday, August 02, 2005

A bit more on eclipses

The moon's orbit is tipped to the plane of the Earth's orbit, which is why we don't have a solar eclipse each month. Usually, when the Moon is new it is above or below the plane containing the Earth and Sun (the ecliptic) and so the tip of its long, thin shadow passes above or below the Earth. Only when the intersection of the planes (the line of nodes) is lined up with the Earth and Sun, and only if the Moon is at the intersection of the two planes at the right time, can we have an eclipse of the Sun. This generally happens twice a year.

On April 8th of this year the tip of the Moon's shadow barely scratched the Earth's surface, like the tip of a rapier across a cheek. This took place in the relatively inaccessible South Pacific and was of brief duration.

On October 13th of this year, the Moon is again in the right position, but the Moon is at a greater than average distance from the Earth and its shadow does not quite reach the Earth's surface. This means that even if you are in the right place (a narrow band across Iberia and northeast Africa) the Moon's disk will not quite cover the Sun's face and the eclipse will be annular -- the Sun will appear as a thin ring of light.