1 00:00:14,000 --> 00:00:17,000 We check the time every day, all day long. 2 00:00:17,000 --> 00:00:20,000 But did you ever wonder - where did telling time come from? 3 00:00:20,000 --> 00:00:22,000 Why does it matter what time it is? 4 00:00:22,000 --> 00:00:24,000 Who determined the clock 5 00:00:24,000 --> 00:00:28,000 and why in the world are there so many different time zones? 6 00:00:28,000 --> 00:00:30,000 The first form of telling time was the sundial 7 00:00:30,000 --> 00:00:33,000 and the earliest sundials known from the archaeological record 8 00:00:33,000 --> 00:00:37,000 are obelisks from nearly 5,000 years ago. 9 00:00:37,000 --> 00:00:40,000 Sundials indicate the time by casting a shadow 10 00:00:40,000 --> 00:00:42,000 onto a surface. 11 00:00:42,000 --> 00:00:47,000 The object that casts the shadow is a stick in the center known as a nomen. 12 00:00:47,000 --> 00:00:50,000 A well-constructed sundial can measure time with remarkable accuracy, 13 00:00:50,000 --> 00:00:56,000 and sundials were used to monitor the performance of clocks until the modern era. 14 00:00:56,000 --> 00:00:58,000 But sundials have their limitations too. 15 00:00:58,000 --> 00:01:01,000 Obviously they require the sun to shine, 16 00:01:01,000 --> 00:01:04,000 so they don't work at all during the night when it's dark. 17 00:01:04,000 --> 00:01:08,000 Many different devices have been used over the years to estimate the passage of time: 18 00:01:08,000 --> 00:01:13,000 candles and sticks of incense that burn down at fairly predictable speeds 19 00:01:13,000 --> 00:01:15,000 have been used, along with the hourglass. 20 00:01:15,000 --> 00:01:18,000 Hourglasses are devices in which fine sand 21 00:01:18,000 --> 00:01:21,000 pours through a tiny hole at a constant rate 22 00:01:21,000 --> 00:01:26,000 and indicates a predetermined passage of an arbitrary period of time. 23 00:01:26,000 --> 00:01:28,000 The origin of the hourglass is uncertain, 24 00:01:28,000 --> 00:01:32,000 although beginning in the 14th century, the hourglass was used commonly, 25 00:01:32,000 --> 00:01:34,000 especially on board ships. 26 00:01:34,000 --> 00:01:37,000 The motion of the boat on the water did not affect the hourglass, 27 00:01:37,000 --> 00:01:41,000 unlike other time-measuring devices. 28 00:01:41,000 --> 00:01:44,000 The mechanical clock was invented in the 13th century 29 00:01:44,000 --> 00:01:48,000 which sparked a big change in traditional timekeeping methods. 30 00:01:48,000 --> 00:01:52,000 This modern clock relied on the swing of a pendulum 31 00:01:52,000 --> 00:01:55,000 or the vibration of a quartz crystal, which was far more accurate than sand 32 00:01:55,000 --> 00:01:57,000 or candles. 33 00:01:57,000 --> 00:02:00,000 Today, the basis for scientific time 34 00:02:00,000 --> 00:02:02,000 is a continuous count of seconds 35 00:02:02,000 --> 00:02:08,000 based on atomic clocks all around the world, known as the international atomic time. 36 00:02:08,000 --> 00:02:10,000 Why does it matter that we keep track of time? 37 00:02:10,000 --> 00:02:13,000 Well, time regulates our daily lives 38 00:02:13,000 --> 00:02:16,000 and makes it possible to accurately communicate with people 39 00:02:16,000 --> 00:02:18,000 all over the world. 40 00:02:18,000 --> 00:02:22,000 Without a time system, we would have many challenges in farming, 41 00:02:22,000 --> 00:02:25,000 social structures, communication, and business. 42 00:02:25,000 --> 00:02:28,000 Take the American railroad system, for example. 43 00:02:28,000 --> 00:02:32,000 In the mid-19th century, each railroad used its own standard time 44 00:02:32,000 --> 00:02:35,000 generally based on the local time of its headquarters, 45 00:02:35,000 --> 00:02:40,000 and the railroad's train schedules were published using its own time. 46 00:02:40,000 --> 00:02:42,000 Some major railroad junctions 47 00:02:42,000 --> 00:02:44,000 served by several different railroads 48 00:02:44,000 --> 00:02:48,000 had a separate clock for each railroad, each showing a different time. 49 00:02:48,000 --> 00:02:52,000 The distance between New York and Boston is about 2 degrees, or 8 minutes, 50 00:02:52,000 --> 00:02:56,000 which can be the difference between making or missing your train connection. 51 00:02:56,000 --> 00:02:59,000 If the difference between New York and Boston 52 00:02:59,000 --> 00:03:02,000 is 8 minutes, imagine the difference 53 00:03:02,000 --> 00:03:04,000 between Boston and Australia. 54 00:03:04,000 --> 00:03:07,000 The use of time zones irons out these differences 55 00:03:07,000 --> 00:03:10,000 and makes communication significantly smoother. 56 00:03:10,000 --> 00:03:12,000 A time zone is a region on earth 57 00:03:12,000 --> 00:03:14,000 that has a uniform standard time. 58 00:03:14,000 --> 00:03:17,000 There are 40 time zones on land 59 00:03:17,000 --> 00:03:19,000 because the earliest and latest time zones 60 00:03:19,000 --> 00:03:21,000 are 26 hours apart. 61 00:03:21,000 --> 00:03:24,000 Any given calendar date exists at some point 62 00:03:24,000 --> 00:03:27,000 on the globe for 50 hours. 63 00:03:27,000 --> 00:03:29,000 So the next time someone asks you 64 00:03:29,000 --> 00:03:31,000 "What time is it?" 65 00:03:31,000 --> 99:59:59,999 Your answer may be a whole lot more complicated than it used to be.