"You see, it was the eclipse. It came into my mind, in the nick of time, how Columbus, or Cortez, or one of those people, played an eclipse as a saving trump once, on some savages, and I saw my chance. I could play it myself, now; and it wouldn't be any plagiarism either, because I should get it in nearly a thousand years ahead of those parties."
- Mark Twain, "A Connecticut Yankee in King Arthur's Court"
A solar eclipse is a predictable astrological phenomenon where the moon passes exactly between the earth and the sun. By a trick of astronomical coincidence, it so happens that from our perspective, the disk of the moon appears to be almost exactly the same size as the disk of the sun. Usually the ecliptic of the moon's orbit takes it slightly above or below the sun, but sometimes earth, moon, and sun are exactly aligned.
When this happens, we get a solar eclipse.
The moon's shadow has two parts, the dark umbra within which the disk of the sun is completely blocked out, and the penumbra, where the disk of the sun is only partly blocked out. It may be useful to imagine these two areas as reverse cones. The umbra is the narrowing part of the moon's shadow obtained where the sun's disk represents the widest part of the cone. The penumbra is the widening part of the moon's shadow obtained where rays from the sun's disk are blocked by the opposite edge of the moon. (If drawing this on paper, draw two lines from the edges of the sun to the opposite rims of the moon, so that they cross before they reach the moon.)
Thus where it intersects with the earth, the penumbra has a diameter of about 4800 kilometres. In contrast, the umbra has a diameter of only about 250 km. Only the areas which fall under the path of the umbra will see a total solar eclipse.
As can be seen, a total solar eclipse will take several hours of passing through the penumbra to reach totality, and then several hours again for the moon's shadow to completely leave the earth. Totality itself lasts only between 3 and 8 minutes.
On average, only two or three total eclipses occur every year. Rare years may have as many as five. Every year that has a total lunar eclipse will also have a total solar eclipse. Most of these fall over water, however, or are otherwise inaccessible to easy viewing. This and the narrow path of the umbra makes it seem as though total solar eclipses were much rarer than they are.
Because the moon's orbit is elliptical, the moon is not always the same distance away from the earth when passing between the earth and the sun. Where the apparent diameter of the moon is smaller than that of the sun, the result is an annular eclipse, so called because a 'ring' of sunlight is still visible around the disk of the moon. The rarest of eclipses is the hybrid eclipse, which passes through the annular stage on its way to true totality.
Solar eclipses, including their type, duration, and location, are completely predictable. NASA offers all this data, along with the detailed eclipse paths of past and future eclipses.
"My idea worked. I wanted to keep things as they were till the eclipse was total, otherwise they would be trying again to get me to dismiss the darkness, and of course I couldn't do it."
- Mark Twain