地球是太阳系从内到外的第三颗行星，也是太阳系中直径、水平和密度最大的类地行星。它也常常被称作世界。英语的地球Earth一词源于古英语及日耳曼语。地球已有44～46亿岁，有一颗天然卫星月球围绕着地球以27.32天的周期旋转，而地球以近24小时的周期自转并且以一年的周期绕太阳公转。迪士尼有同名纪录片。
Why the Earth Is Round
地球为何是圆的？
Our never-thought to ask question for today is: Why is the earth round? Why not a cube? A cube? No,that's silly. Is it? Why?Because,because really big things like to be round for some reason. Exactly right.And that reason is gravity.
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.
Since gravity works equally on everything, there would be a tendency to pull those uneven places down. The surface would even itself out. The only geometrical object in which every point on the surface is the same distance from the center is a sphere.
But wait a minute,I've seen pictures of asteroids that have all sorts of crazy shapes.Why aren't they round?only because they arent big enough for gravity to be a determining factor. If an asteroid were to suddenly become as big as a moon, it would indeed rearrange itself gradually into a sphere.
今天，大家从未想过的问题是：地球为何是圆的？为何不是方形的？方形？那太可笑了。是吗？为何？由于由于很大的东西一般都是圆的。完全正确。这个缘由就是重力。
大家如此想，假设地球是一个立方体。而地球表面的某些端点离中间或其它端点比较远。譬如，立方体的每个角离得非常远。
因为重力对所有物质有哪些用途是相等的，因此不均匀的地方会被向下拉。表面使其本身变得平均。唯一一个表面上的每一个角到中心距离都相等的几何物体就是球体。
等一下，我看过不少行星的图片，它们都是奇形怪状的。它们为何不是圆的？
