Well, by counting the number of isotopes in a sample of rock that's undergone radioactive decay, 通过对岩石样品中正经历着放射衰变的同位素计数，

　　geologists have estimated the Earth's birthday, 地质学家们已经估算出了地球的生日，

　　when it first formed from a solar nebula, to be 4.6 billion years ago. 它从太阳星云中诞生大约是在46亿年之前。

　　But just how long is that really? 但是那到底是多久呢？

　　Here's some analogies that might help you understand. 这里有几个类比应该会帮助我们理解。

　　For example, let's imagine the entire history of Earth until the present day as a single calendar year. 例如，我们把地球到现在的整个历史想象成一个日历年。

　　On January 1st, the Earth begins to form. 在1月1号的时候，地球诞生了。

　　By March 3rd, there's the first evidence of single-celled bacteria. 到了3月3号，开始有证据证明单细胞细菌的出现。

　　Life remains amazingly unicellular until November 11th 直到10月11号之前，生物仍惊人地保持着单细胞的状态，

　　when the first multicellular organisms, known as the Ediacaran fauna, come along. 这时第一种多细胞生物也就是埃迪卡拉动物群出现了。

　　Shortly thereafter, on November 16th at 6:08 p. m. is the Cambrian Explosion of life, 不久之后的11月16号下午6点08分，寒武纪生命大爆发，

　　a major milestone, when all of the modern phyla started to appear. 它是一个重要的里程碑，现代生物门类开始出现。

　　On December 10th at 1:26 p. m. , the dinosaurs first evolve but are wiped out by an asteroid just two weeks later. 12月10号下午1点26分，恐龙首先发展起来，但仅仅在两周后就被一个小行星彻底抹去。

　　On December 31st, the mighty Roman empire rises and falls in just under four seconds. 12月31号，强势的罗马帝国崛起又衰落，仅仅用了4秒钟。

　　And Columbus sets sail for what he thinks is India at three seconds to midnight. 哥伦布在他认为是印度三秒到午夜的时候扬帆起航。

　　If you try to write the history of the Earth using just one page per year, 如果你想要把地球历史写下来，且一页纸便是一年，

　　your book would be 145 miles thick, more than half the distance to the international space station. 那么这本书将会有145英里（约233公里）那么厚，比我们到国际空间站的一半距离还远。

　　The story of the 3.2 million year-old Australopithecine fossil known as Lucy would be found on the 144th mile, 320万岁南方古猿化石露西的故事将在第144英里处被找到，

　　just over 500 feet from the end of the book. 仅仅还有500英尺便到了书的结尾。