How Far the Moon?

It's amazing to realize how much the ancient peoples knew about the Earth and the heavens. They knew the Earth is round and just how big it is. They even calculated the distance to the Moon! And they had no sophisticated equipment; they only used their minds.

Here's how they figured out how far away the Moon is: they knew that a solar eclipse is caused by the Moon blocking the light from the Sun. Sometimes it blocks all the light, and the eclipse is called total. If it blocks only a portion of the Sunšs light, it is called partial.

In 190 B.C. there was an eclipse of the Sun. It was total in Aswan, Egypt, but in Alexandria, 500 miles to the north, it was only partial.

To see how this is enough to figure out the distance to the Moon, think of going a ball game. If a tall person sitting in front of you blocks your view of home plate, you have to move a little to the left or right so you can see the plate.

If the blocking person is far away, you hardly have to move at all to see the plate, but if he is just in front of you, you have to move quite a bit. How far you have to move depends on how far away he is. So if you know how far you have to move, you can figure out how far away he is.

The Greek astronomer Hipparchus used just this type of reasoning. It was hard to tell exactly how much of the Sun he was seeing in Alexandria where the eclipse was only partial - how much the Moon was blocking the Sun - (after all, the Sun is pretty bright.) But using his best guess, he estimated that the Moon was 300,000 miles away. The modern value is 250,000 miles, but so much sophisticated equipment is needed. All Hipparchus used was his mind - an "instrument" we all posses.

(09/03/08)