Day362 The Moon 月球

Day362 The Moon 月球

We find that the moon is about 239,000 miles (384,551 km) away from the earth, and, to 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 very 27.3 days. It is our nearest neighbour in space, and like ourselves it is kept tied to the earth by the earth's gravitational pull.

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 is only 2,160 miles (3,389 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. Artists 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 bright. This shows that the moon gives no light of its own. It merely reflects the light of the sun, like a huge mirror hung in the sky.

Yet the dark part of the moon's surface is not absolutely black; generally it is just light enough for us to be able to see its outline, 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 that the surface of the earth or of snow, or even of a wet road, may reflect uncomfortable much of the sun's light on to our faces. In the same way the surface of the whole earth reflects enough of the sun's light on to the face of the moon for us to be able to see the parts of it which would otherwise be dark.

月球

我们发现月球距离地球约23万9千英里（38万4千551公里）。月球与地球总保持这个距离，变化不超过两三千英里。虽然它离地球的距离仍然是那么远，但是它运转的方向却不断地在变。我们发现月球环绕地球的轨道总是圆周形--或很近似圆周形。每一个月，或者更确切些说是每27又1/3天就环绕地球转一圈。在太空中月球是我们最近的邻居。正像我们本身摆脱不了地心吸引力一样，月球也摆脱不了地球的吸引力。

除了太阳而外，月球好像天空中最大的天体了。可是实际上它却是最小的天体之一。只是因为它距离我们太近了，所以看起来才显得大。月球的直径只有2160英里（3389公里）仅仅比地球直径的1/4稍多一点。

每一个月或者更精确些说每29又1/2天有一次有个我们称之为"望月"的时候。这个时候整个月亮的园盘看起来很明亮。在其他一些时候，月亮的园盘只有一部分看起来觉得亮。我们总是发现这亮的部分是面向着太阳的、面背着太阳的那一部分看起来就黑暗。如果画家们能记住这一点，即只有被太阳照射得发高的那部分月球是明亮的，那他们作起画来就会画得更好。这就表明了月球本身是下发光的。月球就像悬在画龙点睛的一面巨型的大镜子一样，它只能反射出太阳光。

但是月球表面上黑暗的部分并不上绝对黑的。一般地这一部分也能有足够的光让我们能勉强刚刚能看清月球的轮廓，这就是我们所说的"新月抱着旧月"的现象。我们用来看到旧月轮廓的光并汪是来自太阳，而是来看地球。我们都清楚地知道，海水的表面、雪地的表面，甚至下雨天的路面都会把很强的太阳光反射到我们的脸上，照得我们很不舒服。同样道理，整个地球的表面也足可以让我们看清月球的轮廓，如果没有地球的反射光，那月球的这一部分将会是黑暗的。