Carol Pataff, age 12, of Staten Island, New York, for her question:

What type of eclipse happens most often?

Eclipses seem to occur at unexpected times. But actually they keep to a rigid schedule of appearances, which repeats after many years. Astronomers count them by the eclipse year, which is about 346 days, and by a longer period called the saros, which is about 18 years. A year may bring two solar and two lunar eclipses, or as many as seven assorted eclipses. In the long run we get more eclipses of the sun, though they appear in different types.

Eclipses are rare events because they depend on the moon's orbit, which is far from perfect. A solar eclipse occurs when the new moon passes directly between us and the sun. This should, you would think, happen every month. But the wobbly lunar orbit usually passes above or below our view of the sun. During a year, at least two new moons may blot out part or all the sun's face, giving us two solar eclipses. During other years Tie may get three or five solar eclipses of various types.

A lunar eclipse occurs when the earth's shadow falls on the face of the full moon. This occurs when the earth happens to pass between the moon and the sun. Sometimes we get no visible lunar eclipse through the year. Another year may bring one, two or as many as three. On the average, we get four solar eclipses to three lunar eclipses.

Though we stet more solar eclipses, more of us get a chance to see a lunar eclipse. This is because a solar eclipse is visible only along a narrow path and a lunar eclipse may be seen for thousands of miles. A solar eclipse is soon over and the next one appears in another part of the globe. A lunar eclipse lasts for several hours and most of North America gets a chance to see it.

Everybody's favorite, of course, is the total solar eclipse. It occurs somewhere on the earth about three times during a two year period. When it happens the moon is in exactly the right position to cover the brilliant sun. Actually the sun is 400 times wider than the moon. But because it is 400 times farther away, they appear to be the same size.

Sometimes the eclipsing moon fails to make a perfect match. Its dark disk takes a round bite but does not cover the sun's bright face completely. This is a partial solar eclipse. When the eclipsing moon is farther from us than usual, it appears smaller. It may center on the sun, and fail to cover a dazzling circle around the rim. This is called an annular eclipse.

Of the solar eclipses, only about 23 per cent are total. Another 35 per cent are partial and another 32 per cent are annular eclipses. Five per cent appear to be both total and annular. During a lunar eclipse, as a rule the earth's great shadow spreads across the entire face of the full moon. But sometimes, when the moon is farther from us than usual, only the tip of the long pointed shadow falls upon it. This sort of lunar eclipse is barely visible.