We find that the moon is about 239，000 miles (384，551 km) away from the earth， and it always remains the same distance without a change of more than a few thousand miles. Although there still is a long distance between the moon and the earth， its direction continually changes.
We find that it is traveling round the earth in a circle—or very nearly a circle， going completely round once a month or more exactly once very 27 1/3 days. It is our nearest neighbor in space， and like ourselves it is kept tied to the earth by the earth’s gravitational① pull.
The moon looks the biggest object in the sky except for the sun. Actually it is one of the smallest. It looks big just because it is so near to us. Its diameter② is only 2，160 miles (3，389 km) which is just a little more than a quarter of the diameter of the earth.
At a time of each month， or more exactly， every 29.5 days， we call “full moon” at which 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 opposite part appears dark. Artists could make their pictures better if they kept this in mind， that is to say， 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， hunging in the sky like a huge mirror.
Yet the dark part of the moon’s surface is not absolutely③ black； generally speaking， it is enough for us to be able to see its outline with the light， that is the saying “the old moon in the new moon’s arms.” The light by which we see the outline of the old moon does not come from the sun， but from the earth.
As we know， the surface of the sea or of snow， or even of a wet road， may reflect much strong sunlight to our faces， making us uncomfortablly. In the same way， the surface of the whole earth reflecting sunlight is enough for us to see the outline of the moon. The part of the moon would be dark if there were no light reflectd from the sun.
If there were any inhabitants④ in the moon， they would also see our earth reflecting the light of the sun， like a huge mirror hunging in the sky. They would speak of earth-light just as we speak of moon-light. “The old moon in the new moon’s arms” is nothing but part of the moon’s surface on which it is night and lighted up by earth light. In the same way， the lunar inhabitants would occasionally⑤ see part of our earth in full sunlight， and the rest is lighted only by moon-light； they might call this “the old earth in the new earth’s arms.”
① gravitationaladj. (万有)引力的，重力的
② diametern. 直径
③ absolutelyadv. 绝对地，完全地
④ inhabitantn. (某地区的)居民，居住者
⑤ occasionallyadv. 偶尔，间或
月球
我们发现月球距离地球约23.9万英里（384，551公里）。月球与地球总会保持这样一个距离，就算有变化也不会超过两三千英里。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。
我们发现月球总是以圆周形轨道——或很近似圆周形——环绕地球运转。月球每个月或更确切些说每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。正如我们摆脱不了地心吸引力一样，月球同样摆脱不了地球的吸引力。
除太阳外，月球好像天空中最大的天体了。实际上它是最小的天体之一，只是它离我们太近，让人觉得它很大。月球的直径仅比地球直径的1/4稍多一点，只有2，160英里（3，389公里）。
每月或者更精确些说每29.5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圆盘看起来会很明亮。其他时候，月亮的园盘只有一部分看起来是亮的。我们会发现这亮的部分面向着太阳、而背着太阳的一部分看起来就是黑暗。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不发光的。月球就像悬在空中的一面大镜子一样，只能反射太阳的光。
但是月球表面上黑暗的部分并不是绝对黑的。一般情况下，仅这一部分的光也能让我们看清月球的轮廓，这就是我们所说的“新月抱着旧月”。我们用来看到旧月轮廓的光并非来自太阳，而是来看地球。
我们都知道，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。
假定月球上也有居民，他们也会看到地球像一面高悬在天空的巨型镜子一样，反射太阳光到月球上。“月球居民们”谈到地球光会和我们地球人谈到月亮光一样。“新月抱旧月”只不过就是月球表面上正处于黑夜的那一部分被地球的反射光反照的结果。同理，月球上的居民偶尔也会看到我们地球的一部分完全处在阳光的照射下，而地球的其余的部分只能被月光照到；月球的居民们或许也会把这种现象称作“新地球抱着旧地球”吧。