我国古代先人非常有智慧，在汉代古书刘安的《淮南子·齐俗》中，就提到过“四方上下谓之宇，往固来今谓之宙”，简单来说宇是空间，宙是时间，所以宇宙学是一门研究时空的学问。
在宇宙中，时间和空间基本上是一回事。因为当你看到宇宙深处的时候，你就看到了过去。我们通常提到的宇宙学长度单位：光年，看起来挺奇怪，像个时间单位，实际上是长度单位，代表是光走一年的距离。
光走一年的距离是多远呢？也不是很远，大概就是94600亿公里。那银河系有多大呢？直径十万光年以上，也就是说，从银河系的这一端走到另外一端，光要走十万年。
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