Welcome to You Ask Andy

Jeff Levitch, age 12, of San Rafael,California, for his question:

How do solar and lunar eclipses differ?

A lunar eclipse occurs when the planet Earth casts its shadow over the face of the full moon. A solar eclipse occurs when the new moon gets directly between us and the sun. Both of these dramatic events depend on the motions of the orbiting moon.

During the lunar month, the orbiting moon travels in a ring around the earth. At one stage of its journey, it passes more or less between us and the sun. We see the thin sliver of the new moon. Then it swings around to the opposite side of its orbit and the earth is between the moon and the sun. Then we see the full golden moon, for the sun is shining on the entire side of the moon that faces the earth.

This is when a lunar eclipse can occur. It can happen because our solid planet has a shadow, a long tapering shadow that points 900,000 miles out into space. Naturally it points away from the sun from the night side of the globe. The average distance of the moon is almost 240,000 miles. So you would expect it to pass through the earth's eclipsing shadow once every month when it passes through the phase of full moon.

However, the lunar orbit is slightly oval and also tilted six degrees to the earth's orbit. As a rule, the full moon passes just above or just below the earth's pointing shadow. Sometimes it does not come within the eclipsing range of our shadow for a year or more and no lunar eclipse occurs. Sometimes we get one or two lunar eclipses during a year and once in a great while there are three.

The solid moon also has a long tapering shadow and to create a solar eclipse, the lunar shadow must be able to reach the earth. This is a problem because the average length of this shadow is quite a bit shorter than the moon's average distance from the earth. Another problem occurs because a solar eclipse depends on a perfect line up, in which the new moon passes exactly between us and the sun. Usually the tilted lunar orbit takes the new moon north or south of the sun.

Nevertheless, there are times when things line up dust right and from a small part of the earth we behold the awesome sight of a total solar eclipse. At this time, the moon happens to be close enough so that its shadow can touch us and travel a long narrow path on the globe. Meantime the dark new moon completely covers the dazzling face of the sun.

The biggest mystery is how our little moon can cover completely the face of the enormous sun. This is explained by sizes, which appear to grow smaller with distance. Actually the sun is 400 times wider than the moon and 400 times farther away. Seen from the earth, the 400 times smaller moon appears the same size as the 400 times more distant sun.

The occurrence of solar and lunar eclipses seems like a very chancy business. But actually the wobbly moon repeats its patterns after about 18 years. During this time, we get a certain series of eclipses. After about 18 years, the series is repeated, this time appearing farther to the west. This cycle enables astronomers to predict both solar and lunar eclipses for years ahead.

 

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