WEBVTT 00:00:13.895 --> 00:00:14.819 When I was a kid, 00:00:14.819 --> 00:00:16.127 my understanding of the seasons 00:00:16.127 --> 00:00:18.124 was that December and January were cold 00:00:18.124 --> 00:00:19.634 and covered with snow, 00:00:19.634 --> 00:00:21.970 April and May were bursting with flowers, 00:00:21.970 --> 00:00:24.571 July and August were hot and sunshiny, 00:00:24.571 --> 00:00:28.770 and September and October were a kaleidoscope of colorful leaves. 00:00:28.770 --> 00:00:30.234 It was just the way the world worked, 00:00:30.234 --> 00:00:31.901 and it was magical. 00:00:31.901 --> 00:00:33.319 If you had told me back then 00:00:33.319 --> 00:00:35.193 that one-third of Earth's population 00:00:35.193 --> 00:00:36.862 had never seen snow 00:00:36.862 --> 00:00:40.741 or that July 4th was most definitely not a beach day, 00:00:40.741 --> 00:00:42.403 I would have thought you were crazy. 00:00:42.403 --> 00:00:45.412 But in reality, seasonal change with four distinct seasons 00:00:45.412 --> 00:00:48.040 only happens in two regions on the planet. 00:00:48.040 --> 00:00:49.044 And, even in those two, 00:00:49.044 --> 00:00:50.583 the seasons are reversed. 00:00:50.583 --> 00:00:51.584 But why? 00:00:51.584 --> 00:00:53.421 A lot of people have heard of an astronomer 00:00:53.421 --> 00:00:54.895 called Johannes Kepler 00:00:54.895 --> 00:00:57.343 and how he proved that planetary orbits are elliptical 00:00:57.343 --> 00:01:00.093 and that the sun is not at the center of the orbit. 00:01:00.093 --> 00:01:02.011 It was a big deal when he figured this out 00:01:02.011 --> 00:01:03.346 several hundred years ago. 00:01:03.346 --> 00:01:05.890 His discovery solved a lot of mathematical problems 00:01:05.890 --> 00:01:07.175 that astronomers were having 00:01:07.175 --> 00:01:09.166 with planetary orbit measurements. 00:01:09.166 --> 00:01:12.318 While it's true that our orbit's not perfectly circular, 00:01:12.318 --> 00:01:14.274 those pictures in our science books, 00:01:14.274 --> 00:01:15.774 on TV, and in the movies 00:01:15.774 --> 00:01:17.113 give an exaggerated impression 00:01:17.113 --> 00:01:19.355 of how elongated our orbit is. 00:01:19.355 --> 00:01:22.741 In fact, Earth's orbit is very nearly a perfect circle. 00:01:22.741 --> 00:01:25.785 However, because Earth's orbit is technically an ellipse, 00:01:25.785 --> 00:01:27.370 even though it doesn't look like one, 00:01:27.370 --> 00:01:29.867 and the sun isn't quite exactly at the center, 00:01:29.867 --> 00:01:31.459 it means that our distance from the sun 00:01:31.459 --> 00:01:33.125 does change through the year. 00:01:33.125 --> 00:01:34.108 Ah-ha! 00:01:34.108 --> 00:01:37.504 So, winter happens when the Earth is further away from the sun! 00:01:37.504 --> 00:01:39.337 Well, no, not so fast. 00:01:39.337 --> 00:01:41.405 The Earth is actually closer to the sun 00:01:41.405 --> 00:01:43.315 in January than we are in July 00:01:43.315 --> 00:01:45.764 by 5 million kilometers. 00:01:45.764 --> 00:01:48.023 January is smack-dab in the middle 00:01:48.023 --> 00:01:49.649 of the coldest season of the year 00:01:49.649 --> 00:01:51.261 for those of us up north. 00:01:51.261 --> 00:01:52.398 Still not convinced? 00:01:52.398 --> 00:01:53.524 How about this: 00:01:53.524 --> 00:01:55.607 Summer and winter occur simultaneously 00:01:55.607 --> 00:01:57.233 on the surface of our planet. 00:01:57.233 --> 00:01:58.572 When it's winter in Connecticut, 00:01:58.572 --> 00:02:00.194 it's summer in New Zealand. 00:02:00.194 --> 00:02:01.822 So, if it's not the distance from the sun, 00:02:01.822 --> 00:02:03.111 what else could it be? 00:02:03.111 --> 00:02:04.533 Well, we need to also need to know 00:02:04.533 --> 00:02:06.325 that the Earth doesn't sit straight up. 00:02:06.325 --> 00:02:07.622 It actually tilts. 00:02:07.622 --> 00:02:09.411 And that axial tilt of the Earth 00:02:09.411 --> 00:02:12.290 is one of the main reasons for the seasons. 00:02:12.290 --> 00:02:14.201 The Earth spins on an axis 00:02:14.201 --> 00:02:17.169 that's tilted 23.5 degrees from vertical. 00:02:17.169 --> 00:02:19.549 At the same time, the Earth revolves around the sun 00:02:19.549 --> 00:02:23.092 with the axis always pointing in the same direction in space. 00:02:23.092 --> 00:02:24.171 Together with the tilt, 00:02:24.171 --> 00:02:26.150 the spinning and revolving causes the number 00:02:26.150 --> 00:02:28.264 of hours of daylight in a region to change 00:02:28.264 --> 00:02:29.434 as the year goes by, 00:02:29.434 --> 00:02:30.935 with more hours in summer 00:02:30.935 --> 00:02:32.948 and fewer in winter. 00:02:32.948 --> 00:02:35.938 So, when the sun is shining on the Earth, it warms up. 00:02:35.938 --> 00:02:38.483 After the sun sets, it has time to cool down. 00:02:38.483 --> 00:02:39.354 So, in the summer, 00:02:39.354 --> 00:02:42.493 any location that's about 40 degrees north of the equator, 00:02:42.493 --> 00:02:44.114 like Hartford, Connecticut, 00:02:44.114 --> 00:02:45.907 will get 15 hours of daylight each day 00:02:45.907 --> 00:02:47.934 and 9 hours of darkness. 00:02:47.934 --> 00:02:50.105 It warms up for longer than it cools. 00:02:50.105 --> 00:02:52.218 This happens day after day, 00:02:52.218 --> 00:02:54.406 so there is an overall warming effect. 00:02:54.406 --> 00:02:56.239 Remember this fact for later! 00:02:56.239 --> 00:02:57.945 In the winter, the opposite happens. 00:02:57.945 --> 00:03:00.033 There are many more hours of cooling time 00:03:00.033 --> 00:03:01.243 than warming time, 00:03:01.243 --> 00:03:04.703 and day after day, this results in a cooling effect. 00:03:04.703 --> 00:03:06.830 The interesting thing is, as you move north, 00:03:06.830 --> 00:03:10.252 the number of daylight hours in summer increases. 00:03:10.252 --> 00:03:13.376 So, Juneau, Alaska would get about 19 hours of daylight 00:03:13.376 --> 00:03:17.603 on the same summer day that Tallahassee, Florida gets about 14. 00:03:17.603 --> 00:03:20.376 In fact, in the summertime at the North Pole, 00:03:20.376 --> 00:03:22.555 the sun never sets. 00:03:22.555 --> 00:03:26.056 OK, then, it's all about daylight hours, I've got it! 00:03:26.056 --> 00:03:29.077 Well, no, there's another important piece to this puzzle. 00:03:29.077 --> 00:03:30.959 If daylight hours were the only thing 00:03:30.959 --> 00:03:32.602 that determined average temperature, 00:03:32.602 --> 00:03:34.516 wouldn't the North Pole be the hottest place 00:03:34.516 --> 00:03:36.163 on Earth in northern summer 00:03:36.163 --> 00:03:38.113 because it receives 24 hours of daylight 00:03:38.113 --> 00:03:40.577 in the months surrounding the summer solstice? 00:03:40.577 --> 00:03:41.910 But it's the North Pole. 00:03:41.910 --> 00:03:43.455 There's still icebergs in the water 00:03:43.455 --> 00:03:44.748 and snow on the ground. 00:03:44.748 --> 00:03:46.431 So, what's going on? 00:03:46.431 --> 00:03:47.373 The Earth is a sphere 00:03:47.373 --> 00:03:49.883 and so the amount of solar energy an area receives 00:03:49.883 --> 00:03:52.343 changes based on how high the sun is in the sky, 00:03:52.343 --> 00:03:54.426 which, as you know, changes during the day 00:03:54.426 --> 00:03:56.168 between sunrise and sunset. 00:03:56.168 --> 00:03:58.672 But, the maximum height also changes during the year, 00:03:58.672 --> 00:04:01.015 with the greatest solar height during the summer months 00:04:01.015 --> 00:04:04.059 and highest of all at noon on the summer solstice, 00:04:04.059 --> 00:04:06.354 which is June 21st in the northern hemisphere 00:04:06.354 --> 00:04:09.176 and December 21st in the southern hemisphere. 00:04:09.176 --> 00:04:10.859 This is because as the Earth revolves, 00:04:10.859 --> 00:04:13.227 the northern hemisphere ends up tilted away 00:04:13.227 --> 00:04:14.904 from the sun in the winter 00:04:14.904 --> 00:04:16.197 and toward the sun in summer, 00:04:16.197 --> 00:04:18.491 which puts the sun more directly overhead 00:04:18.491 --> 00:04:20.334 for longer amounts of time. 00:04:20.334 --> 00:04:23.205 Remember those increased summer time daylight hours? 00:04:23.205 --> 00:04:26.334 And solar energy per square kilometer increases 00:04:26.334 --> 00:04:28.376 as the sun gets higher in the sky. 00:04:28.376 --> 00:04:30.340 So, when the sun's at an angle, 00:04:30.340 --> 00:04:31.754 the amount of energy delivered 00:04:31.754 --> 00:04:34.960 to each square of the sunlit area is less. 00:04:34.960 --> 00:04:37.840 Therefore, even though the North Pole is getting 24 hours 00:04:37.840 --> 00:04:39.220 of daylight to warm up, 00:04:39.220 --> 00:04:41.850 the sunlight it receives is very spread out 00:04:41.850 --> 00:04:44.979 and delivers less energy than a place further south, 00:04:44.979 --> 00:04:46.604 where the sun is higher in the sky 00:04:46.604 --> 00:04:48.856 because it's more tilted toward the sun. 00:04:48.856 --> 00:04:51.027 Besides, the North Pole has a lot to make up for. 00:04:51.027 --> 00:04:53.569 It was cooling down without any sunlight at all 00:04:53.569 --> 00:04:54.944 for 6 months straight. 00:04:54.944 --> 00:04:57.698 So, as the seasons change, wherever you are, 00:04:57.698 --> 00:05:00.660 you can now appreciate not just the beauty of each new season 00:05:00.660 --> 00:05:02.424 but the astronomical complexity 00:05:02.424 --> 00:05:04.621 that brings them to you.