Peter Walsh Jr., age 12, of Westbrook, Me., for his question:
WHY ARE ECLIPSES SO RARE?
Every eclipse is different. No two are alike. In a total eclipse of the moon, the color and brightness will depend on the conditions in the earth's atmosphere. If there's much dust in the atmosphere, the eclipsed moon glows with deep red. If the weather is stormy or clouds fill the twilight zone, only a small amount of light will be scattered to the moon by the earth's atmosphere and the eclipse will look dark.
The earth, moon and sun move in predetermined orbits, and as a result of this carefully arranged pattern, only a small number of eclipses are seen on the moon as the earth comes between it and the sun, and on the sun as the moon comes between it and the earth.
Eclipses of the moon usually take place two times each year, about five or six months apart. The shadow of the earth is about 5,700 miles in diameter at the distance of the moon, and the moon takes a maximum of three hours and 40 minutes to pass completely through the shadow.
When the moon moves around to the opposite side, the sun itself is eclipsed. A dark, cone shaped shadow, called the umbra, stretches behind the moon, producing a small black spot on the surface of the earth. Only people within the spot see a total eclipse, whereas a lunar eclipse can be seen, weather permitting, by all of the people on the hemisphere facing the moon.
In a fixed location, at least one eclipse of the moon occurs each year, but the average time between total eclipses of the sun is 300 years. To be sure of seeing a particular solar eclipse, a person must travel to the area where the shadow falls.
The black spot on earth of an eclipse of the sun is 100 miles or so across. Its exact size and position can be computed beforehand for each eclipse.
Eclipses of the sun vary. Sometimes the shadow cone sweeps rapidly across the polar regions while at other times it moves more slowly across the tropics.
The black spot of a sun eclipse moves along the path of totality from the sunrise edge of the earth to the sunset edge in about tour hours. Sometimes the end of the umbral cone does not quite reach the surface of the earth because the moon is too far away. A total eclipse cannot take place under these circumstances.
In ancient days, eclipses were thought to be the mysterious workings of the gods.