How do we know how far it is to the Moon?

   Long, long ago men watched the sky and tried to figure out ways of measuring the things they saw mere. How far was it to the Moon, for example? These ancient sky watchers were good at mathematics. They did some rather complicated figuring, and they decided that the Moon was about 240,000 miles away.
   Today sky watchers have a way of testing the figures for us. They do it with radar. They broadcast radio waves which travel to the Moon and then bounce back to earth. The radar measuring instruments tell how many seconds it takes the waves to make the round trip. Divide this by two and we know how many seconds it takes the waves just to go to the Moon. Since we know how fast radio waves travel, we have only to do some multiplying. We multiply the speed of the waves by the number of seconds it takes for them to reach the Moon. The answer is — about 240,000 miles. That's how far you will have to travel when you go by rocket to the Moon.