The ancient Greeks did it, and you can too. Find out how some easily observable facts allow you to measure the approximate distance from the Earth to the Moon.

One of the hardest parts of calculating distances in space is the difficulty finding reference points. The size or distance of objects on Earth can be hard to estimate, but they occupy a landscape which can be measured, providing a jumping-off point. The moon gives up a few clues — it's clearly closer than the Sun or the stars, but it's still drifting in a nothingness that's hard to measure.

The distance to the moon was measured, or at least approximated, over 2000 years ago, by our old friends, the Greeks. They'd already figured out the circumference and consequently the diameter of the earth, providing the one absolute number on which to base the rest. After that, it's geometry.

Many people have held up a round object and let it block the Sun. Most of the time, it's not an exact fit. A slice of Sun peeks through, or a little of the surrounding area is blocked out. When a round object is held up in front of the Sun, it creates a cone of darkness that tapers down to one point. At that one point, the object blocks out all of the Sun, and nothing else. That point, on Earth, is 108 times the diameter of the object. A beach ball will create a shadow 108 beach balls long, which at the farthest point will block out the Sun completely. A penny will create a shadow 108 pennies long. The Earth will create a shadow 108 Earth diameters long.

The Moon passes within that shadow during a lunar eclipse. So no matter how big or small the Moon is, it had to pass within 108 Earth diameters of the Earth. In fact, during lunar eclipses, it was observed that the Moon was imperfectly blocked by the shadow of the Earth. The shadow was roughly 2.5 times the width of the Moon.