阅读理解

　　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 2160 miles(3389 km), or a little more than a quarter of the diameter of the earth．

　　Once a month, or, more exactly, once every 29.5 days, at the time we call “full moon”, its whole disc 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．People could make their pictures better if they kept this in mind－only those parts of the moon which are lighted up by the sun are brighter．This shows that the moon gives no light of its own．It only throws back the light of the sun, like a huge mirror hung in the sky．

　　Yet the dark part of the moon’s surface is not completely black; usually it is just light enough for us to be able to see its shape, so that we speak of seeing “the old moon in the new moon’s arms”．The light by which we see the old moon does not come from the sun, but from the earth．We know well how the surface of the sea or of snow, or even of a wet road, may throw back uncomfortably much of the sun’s light on to our faces．In the same way the surface of the whole earth throws back enough of the sun’s light on to the face of the moon for us to be back to see the parts of it which would otherwise be dark．