单项选择题

We find that the moon is about 239,000 miles away from the earth, and that within a few thousand miles, its distance always remains the same. Yet a very little observation shows that the moon is not standing still. Its distance from the earth remains the same, but its direction continually changes. We find that it is traveling in a circle — or very nearly a circle — round the earth, going completely round once a month, or more exactly, once every days. It is our nearest neighbor in space, and like ourselves it is kept tied to the earth by the earth’s gravitational pull (地心吸力). Except for the sun, the moon looks the biggest object in the sky. Actually it is one of the smallest, and only looks big because it is so near to us. Its diameter (直径) is only 2,160 miles, or a little more than a quarter of the diameter of the earth. Once a month, or, more exactly, once every days, at the time we call "full moon", its whole disc (圆面) looks bright. At other times only part of it appears bright, and we always find that this is the part which faces towards the sun, while the part facing away from the sun appears dark. This shows that the moon gives no light of its own. It merely reflects the lights of the sun, like a huge mirror hung in the sky.

The diameter of the earth is about ______ .