Juanita Davies, age 13, of Calgary, Alberta, Canada, for her question:

How can we prove that the earth is rotating?

We space agers take this for granted. When very young, we are told that the earth rotates on its axis, once every calendar day. This information seems reasonable. However, when a person starts thinking about it and looking at the evidence, it seems just as reasonable to assume that the earth does not rotate. Instead, the starry sky could be revolving around a stationary planet. This is just what almost everybody thought to be true until quite recently in human history.

The motions of the earth have challenged the best brains in history since ancient times. Our cavemen ancestors no doubt assumed that the earth is flat and that every day the sky wheels overhead. The scholars of ancient Greece, however were not likely to be fooled by appearances. Several of these great men fissured that the earth is a sphere. They also reasoned that the heavenly parade rises and sets as the round globe rotates. Other scholars of that time did not agree. And sad to say, their notion of a flat, non rotating; planet prevailed through 14 centuries.

Then in the 1500s, the Polish astronomer Copernicus devised a new model of the Solar System. He suggested that it is just as reasonable to assume that the round earth rotates and the planets revolve around the sun. In the 1600s, the great Galileo of Italy used a telescope to gather masses of evidence in support of the Copernicus theory. We know nova that these men were correct. But at the time, few people accepted the notion    perhaps because it shattered their notion that the earth is the center of the universe.

It was centuries before everybody accepted the idea of a rotating earth and centuries before it was proved to be true. In the meantime, assuming a round, rotating earth, astronomers made many accurate observations and Sir Isaac Newton even figured out the laws of gravitation. However, without proof, one argument was hard to refute. If the earth rotates, surely we should feel it moving. Mathematicians explained that we feel only acceleration and deceleration. If the earth rotates at a steady rate, we would not feel the motion    and this factor led to the final proof.

A hanging pendulum is not influenced by the motions of the ground below it. In the 1850s, the French physicist Leon Foucault used this fact to prove conclusively that the earth rotates. His pendulum was a 56 pound bronze ball with a spike on the under¬side. It was suspended on a steel wire from inside the immense dome of the Pantheon building in Paris. The public was invited to see the dramatic demonstration. The pendulum could be trusted to hang straight down. But if the earth rotates, the ground below it would move    and the spike on the ball was cunningly arranged to trace a line in a bed of sand. And trace it did, line after line. This proved conclusively that the round earth rotates on its axis toward the east. We can watch the sun rise and can also find other evidence that the earth rotates. But Foucault's pendulum pro¬vided the indisputable proof that all this evidence is true.

The earth's surface rotation is fastest at the equator and dwindles to zero at the poles. Foucault's pendulum clocked the rotation speed at Paris. This, naturally, was slower than the equatorial speed. When the test was repeated in London, farther north, the rotation speed was even slower. However, the Foucault pendulum proved ro¬tation and was used later to verify the different speeds along latitudes between the equator and the poles.