9:59:59.000,9:59:59.000 How old is the Earth? 9:59:59.000,9:59:59.000 Well, by counting the number of isotopes 9:59:59.000,9:59:59.000 in a sample of rock 9:59:59.000,9:59:59.000 that's undergone radioactive decay, 9:59:59.000,9:59:59.000 geologists have estimated the Earth's birthday, 9:59:59.000,9:59:59.000 when it first formed from a solar nebula, 9:59:59.000,9:59:59.000 to be 4.6 billion years ago. 9:59:59.000,9:59:59.000 But just how long is that really? 9:59:59.000,9:59:59.000 Here's some analogies 9:59:59.000,9:59:59.000 that might help you understand. 9:59:59.000,9:59:59.000 For example, let's imagine the entire history of Earth 9:59:59.000,9:59:59.000 until the present day 9:59:59.000,9:59:59.000 as a single calendar year. 9:59:59.000,9:59:59.000 On January 1st, the Earth begins to form. 9:59:59.000,9:59:59.000 By March 3rd, there's the first evidence 9:59:59.000,9:59:59.000 of single-celled bacteria. 9:59:59.000,9:59:59.000 Life remains amazingly unicellular until November 11th 9:59:59.000,9:59:59.000 when the first multicellular organisms, 9:59:59.000,9:59:59.000 known as the Ediacaran fauna, come along. 9:59:59.000,9:59:59.000 Shortly thereafter, on November 16th at 6:08 p.m. 9:59:59.000,9:59:59.000 is the Cambrian Explosion of life, 9:59:59.000,9:59:59.000 a major milestone, 9:59:59.000,9:59:59.000 when all of the modern phyla started to appear. 9:59:59.000,9:59:59.000 On December 10th at 1:26 p.m., 9:59:59.000,9:59:59.000 the dinosaurs first evolve 9:59:59.000,9:59:59.000 but are wiped out by an asteroid 9:59:59.000,9:59:59.000 just two weeks later. 9:59:59.000,9:59:59.000 On December 31st, the mighty Roman empire 9:59:59.000,9:59:59.000 rises and falls in just under four seconds. 9:59:59.000,9:59:59.000 And Columbus sets sail 9:59:59.000,9:59:59.000 for what he thinks is India 9:59:59.000,9:59:59.000 at three seconds to midnight. 9:59:59.000,9:59:59.000 If you try to write the history of the Earth 9:59:59.000,9:59:59.000 using just one page per year, 9:59:59.000,9:59:59.000 your book would be 145 miles thick, 9:59:59.000,9:59:59.000 more than half the distance 9:59:59.000,9:59:59.000 to the international space station. 9:59:59.000,9:59:59.000 The story of the 3.2 million year-old 9:59:59.000,9:59:59.000 Australopithecine fossil known as Lucy 9:59:59.000,9:59:59.000 would be found on the 144th mile, 9:59:59.000,9:59:59.000 just over 500 feet from the end of the book. 9:59:59.000,9:59:59.000 The United States of America's Declaration of Independence 9:59:59.000,9:59:59.000 would be signed in the last half-inch. 9:59:59.000,9:59:59.000 Or if we compared geologic time 9:59:59.000,9:59:59.000 to a woman stretching her arms 9:59:59.000,9:59:59.000 to a span of six feet, 9:59:59.000,9:59:59.000 the simple act of filing her nails 9:59:59.000,9:59:59.000 would wipe away all of recorded human history. 9:59:59.000,9:59:59.000 Finally, let's imagine the history of the Earth 9:59:59.000,9:59:59.000 as your life -- 9:59:59.000,9:59:59.000 from the moment you're born 9:59:59.000,9:59:59.000 to your first day of high school. 9:59:59.000,9:59:59.000 Your first word, 9:59:59.000,9:59:59.000 first time sitting up, 9:59:59.000,9:59:59.000 and first time walking 9:59:59.000,9:59:59.000 would all take place while life on Earth 9:59:59.000,9:59:59.000 was comprised of single-celled organisms. 9:59:59.000,9:59:59.000 In fact, the first multicellular organism 9:59:59.000,9:59:59.000 wouldn't evolve until you were 12 years old 9:59:59.000,9:59:59.000 and starting 7th grade, 9:59:59.000,9:59:59.000 right around the time 9:59:59.000,9:59:59.000 your science teacher is telling the class 9:59:59.000,9:59:59.000 how fossils were formed. 9:59:59.000,9:59:59.000 The dinosaurs don't appear 9:59:59.000,9:59:59.000 until three months into 8th grade 9:59:59.000,9:59:59.000 and are soon wiped out right around spring break. 9:59:59.000,9:59:59.000 Three days before 9th grade begins, 9:59:59.000,9:59:59.000 when you realize summer is over 9:59:59.000,9:59:59.000 and you need new school supplies, 9:59:59.000,9:59:59.000 Lucy, the Australopithecine, is walking around Africa. 9:59:59.000,9:59:59.000 As you finish breakfast 9:59:59.000,9:59:59.000 and head outside to catch your bus 9:59:59.000,9:59:59.000 44 minutes before school, 9:59:59.000,9:59:59.000 the Neanderthals are going extinct throughout Europe. 9:59:59.000,9:59:59.000 The most recent glacial period 9:59:59.000,9:59:59.000 ends as your bus drops you off 9:59:59.000,9:59:59.000 16 minutes before class. 9:59:59.000,9:59:59.000 Columbus sets sail 50 seconds before class 9:59:59.000,9:59:59.000 as you're still trying to find the right classroom. 9:59:59.000,9:59:59.000 The Declaration of Independence is signed 9:59:59.000,9:59:59.000 28 seconds later 9:59:59.000,9:59:59.000 as you look for an empty seat. 9:59:59.000,9:59:59.000 And you were born 1.3 seconds before the bell rings. 9:59:59.000,9:59:59.000 So, you see, the Earth is extremely, 9:59:59.000,9:59:59.000 unbelievably old 9:59:59.000,9:59:59.000 compared to us humans 9:59:59.000,9:59:59.000 with the fossil record 9:59:59.000,9:59:59.000 hiding incredible stories to tell us about the past 9:59:59.000,9:59:59.000 and possibly the future as well. 9:59:59.000,9:59:59.000 But in the short time we've been here, 9:59:59.000,9:59:59.000 we've learned so much 9:59:59.000,9:59:59.000 and will surely learn more 9:59:59.000,9:59:59.000 over the next decades and centuries, 9:59:59.000,9:59:59.000 near moments in geological time.