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How old is the Earth?
地球的年齡有多大？
Well, by counting the number of isotopes in a sample of rock that's undergone radioactive decay, geologists have estimated the Earth's birthday, when it first formed from a solar nebula, to be 4.6 billion years ago.
通過對巖石樣品中正經歷著放射衰變的同位素計數，地質學家們已經估算出了地球的生日，它從太陽星云中誕生大約是在46億年之前。
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.
這里有幾個類比應該會幫助我們理解。
For example, let's imagine the entire history of Earth until the present day as a single calendar year.
例如，我們把地球到現在的整個歷史想象成一個日歷年。
On January 1st, the Earth begins to form.
在1月1號的時候，地球誕生了。
By March 3rd, there's the first evidence of single-celled bacteria.
到了3月3號，開始有證據證明單細胞細菌的出現。
Life remains amazingly unicellular until November 11th when the first multicellular organisms, known as the Ediacaran fauna, come along.
直到10月11號之前，生物仍驚人地保持著單細胞的狀態，這時第一種多細胞生物也就是埃迪卡拉動物群出現了。
Shortly thereafter, on November 16th at 6:08 p. m. is the Cambrian Explosion of life, a major milestone, when all of the modern phyla started to appear.
不久之后的11月16號下午6點08分，寒武紀生命大爆發，它是一個重要的里程碑，現代生物門類開始出現。
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號下午1點26分，恐龍首先發展起來，但僅僅在兩周后就被一個小行星徹底抹去。
On December 31st, the mighty Roman empire rises and falls in just under four seconds.
12月31號，強勢的羅馬帝國崛起又衰落，僅僅用了4秒鐘。
And Columbus sets sail for what he thinks is India at three seconds to midnight.
哥倫布在他認為是印度三秒到午夜的時候揚帆起航。
If you try to write the history of the Earth using just one page per year, your book would be 145 miles thick, more than half the distance to the international space station.
如果你想要把地球歷史寫下來，且一頁紙便是一年，那么這本書將會有145英里（約233公里）那么厚，比我們到國際空間站的一半距離還遠。
The story of the 3.2 million year-old Australopithecine fossil known as Lucy would be found on the 144th mile, just over 500 feet from the end of the book.
320萬歲南方古猿化石露西的故事將在第144英里處被找到，僅僅還有500英尺便到了書的結尾。
