It's an essential question that humans have been wondering about for centuries. 这是个很重要的问题，几百年来，人们一直在探索。

　　But the journey toward an answer has been a strange one. 但这段探索之旅的结果却十分奇怪。

　　If you wanted to know the answer to this question in third century B.C.E. Greece, 如果你想知道公元三世纪希腊人给出的答案，

　　you might look up at the night sky and trust what you see, 你就要问亚里士多德了，

　　that's what Aristotle, THE guy to ask back then, did. 他的回答是仰望星空，相信自己的所见。

　　He thought that since we're on Earth, looking up, it must be the center, right? 他想：既然我们从地球向天上看，那么地球一定就是宇宙的中心，不是吗？

　　For him, the sphere of the world was made up of four elements: Earth, water, air, and fire. 他认为，宇宙是由四种元素组成的：土，水，空气和火。

　　These elements shifted around a nested set of solid crystalline spheres. 这些元素在一套嵌套的固体球核周围流动。

　　Each of the wandering stars, the planets, had their own crystal sphere. 每个浮于夜空的恒星和行星都有它们自己的球核。

　　The rest of the universe and all of its stars were on the last crystal sphere. 宇宙剩下的部分和所有星星在最后的球核上。

　　If you watch the sky change over time, 如果你观察天空随时间的变化，

　　you could see that this idea worked fine at explaining the motion you saw. 就可以发现这个理论很好得解释了你所看到的变化。

　　For centuries, this was central to how Europe and the Islamic world saw the universe. 几个世纪以来 ，它就是欧洲和伊斯兰世界的宇宙观核心。

　　But in 1543, a guy named Copernicus proposed a different model. 但是在1543年，一位名叫哥白尼的人提出了一个不同的模型。

　　He believed that the sun was at the center of the universe. 他认为太阳才是宇宙的中心。

　　This radically new idea was hard for a lot of people to accept. 这个十分激进的新思想让大多数人难以接受。

　　After all, Aristotle's ideas made sense with what they could see, and they were pretty flattering to humans. 毕竟亚里士多德的理论在人们眼中才符合事实，它对人类更有吸引力。

　　But a series of subsequent discoveries made the sun-centric model hard to ignore. 但是随后的一系列发现让日心说无法被忽视。

　　First, Johannes Kepler pointed out that orbits aren't perfect circles or spheres. 首先，约翰尼斯·开普勒指出行星运动的轨道并不是完美的圆弧或者球型。

　　Then, Galileo's telescope caught Jupiter's moons orbiting around Jupiter, totally ignoring Earth. 其次，伽利略的望远镜发现木星也有围绕自己的卫星，完全与地球无关。

　　And then, Newton proposed the theory of universal gravitation, 接着牛顿提出了万有引力定律，

　　demonstrating that all objects are pulling on each other. 说明所有的物体都互相吸引。

　　Eventually, we had to let go of the idea that we were at the center of the universe. 最终， 我们不得不放弃地球是宇宙中心的想法。