WEBVTT 1 00:00:00.350 --> 00:00:02.558 Why is summer warm and winter cold? 2 00:00:02.558 --> 00:00:06.779 By the end of this video, you'll know the real reason, and why the 3 00:00:06.779 --> 00:00:08.862 most popular answer is wrong. 4 00:00:09.212 --> 00:00:13.981 Many people think summer is warm because Earth is closer to the Sun. 5 00:00:13.981 --> 00:00:16.544 It sounds reasonable, but it's wrong. 6 00:00:16.544 --> 00:00:21.004 In fact, Earth is closest to the Sun in early January, right in the middle 7 00:00:21.004 --> 00:00:23.342 of the Northern Hemisphere's winter. 8 00:00:23.692 --> 00:00:25.745 Earth's orbit is almost a circle. 9 00:00:25.745 --> 00:00:30.639 At its closest, around January 3, Earth is about 147 million 10 00:00:30.639 --> 00:00:32.629 kilometres from the Sun. 11 00:00:32.629 --> 00:00:37.044 At its farthest, around July 4, it's about 152 million. 12 00:00:37.044 --> 00:00:41.311 That's only a 3% difference, far too small to explain the seasons. 13 00:00:41.661 --> 00:00:45.119 And if distance were the cause, both hemispheres would have 14 00:00:45.119 --> 00:00:46.550 summer at the same time. 15 00:00:46.550 --> 00:00:47.206 They don't. 16 00:00:47.206 --> 00:00:50.802 When it's summer in Canada, it's winter in Australia. 17 00:00:51.152 --> 00:00:52.896 Here's the real reason: 18 00:00:52.896 --> 00:00:56.006 Earth's axis is tilted about 23.4 degrees. 19 00:00:56.006 --> 00:01:00.708 As Earth orbits, the axis keeps pointing the same way in space, 20 00:01:00.708 --> 00:01:02.377 toward the North Star. 21 00:01:02.377 --> 00:01:06.414 So for half the year the Northern Hemisphere leans toward the Sun, 22 00:01:06.414 --> 00:01:08.911 and for the other half it leans away. 23 00:01:09.261 --> 00:01:10.932 Tilt changes two things. 24 00:01:10.932 --> 00:01:13.994 First, how directly sunlight hits the ground. 25 00:01:13.994 --> 00:01:18.186 A high Sun shines straight on, concentrating energy on a small 26 00:01:18.186 --> 00:01:18.530 area. 27 00:01:18.530 --> 00:01:22.990 A low Sun spreads the same beam over a much larger area, so each 28 00:01:22.990 --> 00:01:24.972 square metre gets less heat. 29 00:01:24.972 --> 00:01:29.844 Picture a flashlight pointed straight down versus at a steep angle. 30 00:01:30.194 --> 00:01:32.311 Second, how long the day lasts. 31 00:01:32.311 --> 00:01:36.407 The hemisphere tilted toward the Sun gets long days and short 32 00:01:36.407 --> 00:01:40.777 nights, so the ground has more hours to warm up and fewer to cool 33 00:01:40.777 --> 00:01:41.118 down. 34 00:01:41.118 --> 00:01:45.723 In Vancouver, a June day has about 16 hours of daylight. 35 00:01:45.723 --> 00:01:47.902 A December day has only about 8. 36 00:01:48.252 --> 00:01:50.189 Four moments mark the year. 37 00:01:50.189 --> 00:01:55.068 At the June solstice, around June 21, the north tilts most toward the 38 00:01:55.068 --> 00:01:55.355 Sun. 39 00:01:55.355 --> 00:01:58.791 At the September equinox, day and night are roughly equal 40 00:01:58.791 --> 00:01:59.466 everywhere. 41 00:01:59.466 --> 00:02:02.507 At the December solstice, the south leans in: 42 00:02:02.507 --> 00:02:05.065 summer in Australia, winter in Canada. 43 00:02:05.065 --> 00:02:09.436 And at the March equinox, it's equal again, and the cycle repeats. 44 00:02:09.786 --> 00:02:11.037 Location matters too. 45 00:02:11.037 --> 00:02:15.792 Near the equator, the Sun stays high all year and day length barely 46 00:02:15.792 --> 00:02:19.899 changes, so seasons are often defined by rainfall instead. 47 00:02:19.899 --> 00:02:22.578 Near the poles, tilt is extreme: 48 00:02:22.578 --> 00:02:26.428 in summer the Sun never sets, the midnight sun, 49 00:02:26.428 --> 00:02:29.880 and in winter it never rises, the polar night. 50 00:02:30.230 --> 00:02:33.008 So, seasons aren't about distance. 51 00:02:33.008 --> 00:02:37.210 Earth's tilted axis changes how directly sunlight strikes each 52 00:02:37.210 --> 00:02:40.654 hemisphere, and how many hours of daylight it gets. 53 00:02:40.654 --> 00:02:43.107 Tilt toward the Sun, and it's summer. 54 00:02:43.107 --> 00:02:45.073 Tilt away, and it's winter.