Earth's Man-made Moons

Since the first man-made moon was launched in 1957 the number has steadily grown, and we are now beginning to realise that the artificial satellite is capable of doing much useful work.

If we want to send a radio message to a country on the other side of the world the problem is complicated by the earth's round shape. Radio waves, like light waves, travel in straight lines. To send radio waves round the world is like trying to see round a corner.

Sometimes, at a street corner, you may have seen a large mirror set up to show drivers and pedestrians if anything is coming from another direction. This is one way of seeing round a corner. Radio waves, too, will bounce off certain things and change their direction.

High up in the earth's atmosphere is a layer of air which will make some radio waves do this, and so a message can be sent round the world. This layer of air deflects the waves back to earth, and prevents them going straight out into space.

At certain times, however, this layer of air may be upset and fail to work properly. It frequently happens, for example, when there are many spots visible on the surface of the sun. A satellite which is high above the earth can be used to bounce radio waves back to another part of the world. Unlike the layer of air which does this, the satellite is not disturbed by sunspots and so it always works well. Television pictures are now sent over long distances by the same method.