INTRODUCTION

It may be impossible to overestimate the importance of the moon to us earth-dwellers. The moon likely provided our first correct intimation of the idea that one body can orbit another. Eratosthenes showed us how to measure the diameter of the earth in the second century (bc), and made a pretty good estimate of it himself. With a refined measurement of the earth's diameter, we could use that number as a baseline to measure the distance from the earth to the moon. Knowing this distance, we did not find the numbers so incredible when we then measured the distances to nearby asteroids; then to Apollo's Chariot and to all the planets of the solar system, using essentially the same method with simple geometry. Knowing the distance to the sun allowed us to calculate distances to nearby stars, using the diameter of the earth's orbit as a measuring stick. It all started with measuring the distance to the moon.

But much more recently, the moon served as a stepping-stone of another kind. It was a nearby body. We might aspire one day to stand on the moon, because it was so close. We did so aspire, and brought it about. With the astronauts standing there, we thought many previously unthinkable things: among them, the long jump to Mars suddenly seemed within our capabilities. We will certainly accomplish that visit also, and this will lead us ever farther. Who can say how far?