英語(yǔ)時(shí)差：地球?yàn)槭裁词菆A的

Don: It's time once again for one of those Moments of Science that we like to call "Never Thought to Ask."Our never thought to ask question for the day is: Why is the earth round? Ya?l: Why is the earth round? I never thought to ask that.

D: Of course not!

Y: It just seems to be one of those things. The sun is round. The moon is round. The earth is round. What else would it be?

D: Why not a cube?

Y: A cube? No, that's silly.

D: Is it? Why?

Y: Because . . . because . . . (frustrated) . . . because really big things like to be round for some reason!

D: Exactly right. And that reason is gravity.

Y: Gravity?

D: Think of it this way. Suppose the earth were a cube. Then some points on its surface would be farther away from the center than others. The corners, for example, would be much farther out than the rest.

Y: I think I see where you're going. Since gravity works equally on everything, there would be a tendency to pull those uneven places down. The surface would even itself out.

D: Right. And the only geometrical object in which every point on the surface is the same distance from the center is. . .?

Y: A sphere! . . . But wait a minute. I've seen pictures of asteroids that have all sorts of crazy shapes. Why aren't they round?

D: Only because they aren't big enough for gravity to be a determining factor. If that asteroid were to suddenly become as big as a moon, it would indeed rearrange itself gradually into a sphere.

Y: Well, I never . . . !