Why we have seasons

0:00- [Presenter] Have you wondered why we have seasons? Why do we have hot summers, for example, and cool winters? Well, here's what I thought was going on. We know the Earth goes around the sun, right? Sometimes the Earth comes closer to the sun, and other times, it'll go farther away.

0:17So I should think when Earth is closer to the sun, it heats up a lot and that's when we would get hot summers, right? And when it is farthest from the sun, it would be the coolest. Hey, that's when we would have winters. And of course, in between, we would have our spring and fall.

0:34But this turns out to be wrong. Why, you ask? For one, even though the distance between the sun and the Earth does change over the entire year. This is a highly exaggerated picture. The real picture would be somewhat like this. You see, the orbit is almost a perfect circle, and so the distance almost stays the same.

1:01And so the Earth pretty much receives the same amount of sunlight throughout the year and so that possibly cannot explain the seasons. But more importantly, if this were true, then every place on our planet would've experienced the same season, right? I mean, at this location, all parts on the Earth would experience winter.

1:23And at this location, all parts on the Earth would've experienced summer. But we know that doesn't happen. We know that when the Northern Hemisphere experiences winter, the Southern hemisphere experiences summer and vice versa. So how do you explain that? So clearly, this is not the right answer.

1:42Okay, so if the Earth pretty much receives the same amount of sunlight throughout the year, how do we explain seasons? The key is to look at the situation from the side. So quick question, what would the picture look like if we could see this orbit from the side?

2:03Well, first, let's make the sizes of the Earth and sun bigger so we can see them better. This is definitely not to scale, but it'll help us see them better. Now what would the situation look like if we could see it from the side? Well, I would think that the situation should look like this.

2:22I mean, the Earth spins on its own axis and it goes around the sun as well. But if this were the case, again, we can't explain the seasons. I mean, let's think about the sunlight hitting the Earth's surface. If you could draw a few rays of light, then notice the Northern Hemisphere receives one, two, three, four, five rays of sunlight and the southern hemisphere also receives one, two, three, four, five rays of sunlight.

2:52No difference over here. And the same thing is true when the Earth comes over here, right? Five rays of sunlight. Five rays of sunlight. So notice, if this were the case, then again, the northern and the southern hemisphere would receive the same amount of sunlight throughout the year and again, it wouldn't explain the seasons.

3:12But turns out that this is not the picture. The actual picture is the Earth's axis is slightly tilted, and that changes everything. Now look at the amount of sunlight that the Northern hemisphere receives. Hey, it only receives one, two, three rays of sunlight and the southern hemisphere, one, two, three, four, five, six, seven.

3:42So you can clearly see because the Earth's axis tilted, the northern hemisphere and the southern hemisphere received different amount of sunlight. Since the northern hemisphere tilted away from the sun, it receives less sunlight. And since the southern hemisphere tilted towards the sun, it receives more sunlight.

4:04But wait, there's more. Near the equator, the sunlight hits almost head on. So the same energy is concentrated into a smaller area and heats the surface more. But as you move towards the poles, look, the sunlight starts hitting at an angle, spreading that same energy over larger and larger area, making it cooler.

4:27Now because the northern hemisphere is tilted away from the sun, look, most of the sunlight it receives is spread out. So it becomes cooler. It experiences a winter. And because the southern hemisphere is tilted towards the sun, look, it receives a lot of direct, more concentrated sunlight.

4:50So it becomes hotter. It experiences a summer. But wait, there's even more. If you now pick any place in the northern hemisphere, then look, it spends less time facing the sun and more time turned away from it. So that's why during the winter, the day times are shorter and the night times are longer.

5:19But of course, in the southern hemisphere, it's the exact opposite. You spend more time facing the sun and less time being away from it. That's why during the summer, the day times are longer and the night times are shorter. And now guess what happens when the Earth comes here about six months later?

5:41Look, the situation has reversed. Can you see? Now the northern hemisphere tilts towards the sun. As a result, look, it receives more sunlight, and therefore, it experiences summer, and the southern hemisphere is now tilted away and so it experiences winter. So what's the reason for the seasons?

6:06Well, the reason is the Earth's axis around which it spins is slightly tilted. That explains it. More the tilt, more extreme the seasons would be. I mean, imagine if the tilt was like this. Whoa. Now the northern hemisphere would have been completely in the dark for months.

6:24Would've experienced extreme winter. And the southern hemisphere would've experienced extreme summer. Completely in the daylight, right? And the opposite would've happened over here. So it's the tilt that explains the seasons. Okay, but what happens when the Earth is over here and over here?

6:42Well, in these locations, the Earth's tilt is sidewards. Can you see that? It's neither tilted towards or away from the sun. As a result, when you draw the ray of light, if you could imagine the ray of light now hitting the northern and the southern hemisphere, equal amount of ray of light would hit the northern and the southern hemisphere.

7:02And so look, both hemispheres now have equal amount of daytime and nighttime. We call this Equinox. And again, look at the extreme case. Even under extreme cases in these locations, look, the northern and the southern hemisphere will experience the same amount of sunlight.

7:21They would have the same amount of daytime and nighttime. They would both experience Equinox. So when we go from here to here, if you look at the northern hemisphere I is going from winter to summer, and therefore, it experiences spring. But the southern hemisphere is going from summer to winter, so it experiences autumn or fall.

7:41And of course, the opposite happens over here. The northern hemisphere going from summer to winter experiences fall over here, and the southern hemisphere experiences the spring over here, and that explains the season. Of course, the last question we could have is how much tilt does the Earth's axis actually have?

7:59It turns out that tilt is about 23.5 degrees. And so long story short, the fact that the Earth's axis of rotation is slightly tilted is the reason for the seasons.