A total solar eclipse—when the moon passes in front of the sun and blocks it completely—is an amazing sight. To see a total solar eclipse, you have to be in just the right spot on the earth. When you look up in the sky at the sun and the moon, you notice a strange coincidence—both look the same size in the sky. Both the sun and the moon look about one-half degree in diameter. Now, they're not really the same size. The sun's diameter is actually 400 times the moon's diameter. But, you must also take into account that the sun is also 400 times farther away from the earth, reducing its apparent size to the same as the moon's. Because of this relationship, when you are standing on the earth, looking up at the two, you must be in a very limited zone to see the moon cover the entire face of the sun. If you were to move a little north, the sun would peek out over the top of the moon; a little south, and the sun shines past the southern limb of the moon. The match is so good that the "path of totality" is never more than 167 miles in diameter, and is usually less. This means that very few people have seen a total eclipse because the shadow only covers a very small area on the earth.

This diagram (wildly out of scale) shows a side view of the alignment. From anywhere in the gray penumbra, you will see some part of the sun shining from behind the moon. The penumbra is the area of partial eclipse. Only from within the tiny area where the dark umbra touches the earth will you see the sun completely covered and witness a total eclipse.

The earth and the moon are not fixed objects. The moon is busy orbiting the earth. The earth is busy orbiting the sun and additionally rotating on its axis. This means that the spot on the earth where the umbra falls is always in motion and actually traces out a path.