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How being Earth's tilt causes seasons. Created by Sal Khan.
Video transcript
In the last video, we talk about how seasons on Earth are not caused by how close Earth is to the Sun, its orbit. We also hint at the fact that it's actually caused by the tilt of the Earth. And so, in this video, I wanna show you how the tilt of the Earth causes the seasons to happen. So let's draw... I'll try to draw as many diagrams as possible here because at least for my brain they help me visualize what's actually going on. So, we can imagine a top view first. So, let's have a top view. That is the Sun right over there. And let me draw Earth's orbit. So Earth's orbit maybe looks something like that. Let me draw it almost - it is almost circular. So I'll draw this something that's pretty close to a circle right over here. Now, I'm gonna draw Earth at different points in its orbit and I will try to depict the tilt of its rotational axis. And obviously this is not drawn anywhere near close to the scale. Earth is much further away from the Sun and much much smaller than the Sun as well. So I'll draw the Earth at that point, and at this point the Earth will be tilted away from the Sun. So, Earth's tilt does not change if you think about the direction - or at least over the course of a year, if we think about relatively small periods of time - it does not change relative to the direction that it's pointing at in the universe. Let me .. I will talk about that in a second. So, let's say right over here we are pointed away from the Sun, so we're up and out of this page. so I will put some perspective of an arrow. Actually it would be more like up and out of this page, so, that's the direction if you were to come straight out of the North Pole, if you were to come straight out of the South Pole, you would go below that circle right over there. And if I wanna draw the same position but if we were looking sideways, along the orbital plane, or the plane of Earth's orbit, so for looking at from that direction, so let me do it this way. For looking at directly sideways, this is the Sun right over here and this is Earth at that position. If I were to draw an arrow pointing straight out of the North Pole, it would look something like this. So this arrow and this arrow, they are both popping straight out of the North Pole. And so, when we talk about the tilt of the Earth, we're talking about the tilt of its orbital axis, kind of this pole that can go straight between the South Pole and the North Pole. The angle between that and the pole that would actually be at a 90° angle or perpendicular to the plane of its orbit, compared to if it was just sraight up and down relative to the plane of the orbit. So, this right here is the angle of Earth's tilt. Let me draw that a little bit bigger just so it becomes a little bit clearer. So, this is the plane of the orbit, we're looking sideways along the plane of the orbit, and this is Earth right over here. My best attempt to draw a circle. That is Earth. Earth does not rotate - its axis of rotation is not perpendicular to the plane of the orbit. So, this is how Earth would rotate if it was. Earth's rotational axis is at an angle to that vertical relative to the plane of its orbit, I guess you could say it. It rotates at an angle like this. So, this would be the North Pole, that is the South Pole, and so it rotates like this. And that angle relative to being vertical with respect to the orbital plane, this angle right here for Earth right now is 24. ... is 20 ... sorry, 23,4°. 23,4° And for talking about relatively short periods of time, like, you know, our life spans, that is constant. But it is actually changing over long periods of time. That is changing between, and these are rough numbers, it is changing between 22,1° and 24,5°, if my sources are correct, but I guess they're rough estimate of what it's changing between. But I wanna make it clear: this is not happening overnight. The period that it takes to go from roughly a 22° angle to a 24,5° angle and back to the 22° angle is 41,000 years. And this long-term change in the tilt, this might have played to some of the long-term climate change, maybe it might have contributed on some level to some of the Ice Ages that have formed over Earth's past, but for the sake of thinking about our annual seasons, you don't have to worry too much, or you don't have to worry at all really about this variation. You really just have to know that it is tilted, and right now it is tilted at an angle of 23,4°. Now, you might say: 'OK, I understand what the tilt is, but how does that change the seasons, the .. of the Northern or that Southern Hemisphere. And to do that, I'm going to imagine the Earth when the Northern Hemisphere is most tilted away from the Sun and when it is most tilted towards the Sun. So remember, this tilt, the direction this arrow points into relative the rest of the universe, if we assume that this tilt, that 23,4°, it's not changing throughout the year, but depending on where it is in the orbit, it's either going to be tilted away from the Sun, as it is in this example right over here, or it will be tilting towards the Sun. I will do the "towards the Sun" in this magenta color. Or it will be tilting towards the Sun. So, six months later when the Earth is over here, relative to the rest of the universe it will be tilted in that same direction, up, out of this page and to the right. So, out of this page and to the right again, just like it was over here, but now that it's on the other side of the Sun, and that makes the tilt a little bit more towards the Sun. If I were to draw it right over here, it is now tilted towards the Sun. And what I want to think about is how much sunlight will different parts of the planet receive, and I'll focus on the Northern Hemisphere, but you can make a similiar argument for the Southern Hemisphere. I wanna think about how much sunlight they receive at when it's tilted away or tilted towards the Sun. Let's think about those two situations. So, first of all let's think about the situation here where we're tilted away from the Sun. So, let me zoom it a little bit. This is the situation where we're tilted away from the Sun. So, if this is the vertical ... I could actually use this diagram, but let me make it. So, we're tilted away from the Sun like this, let me use a different color... So, if we had an arrow coming straight out of the North Pole, it would look like this. If we have an arrow coming straight out of the North Pole and we're rotating around like that. Out of the page on the left-hand side and into the page on the right-hand side. And so we're rotating towards the east constantly. So, this arrow is in the direction of the east. So, when we're at this point in Earth's orbit, and actually let me copy and paste this and I'm going to use the same exact diagram for the different seasons. So, let me copy and paste this exact same diagram, I'll do it over here for two different points. So, when we're here in Earth's orbit, where is the sunlight coming from? Well, it's going to be coming from the left, at least the way I've drawn the diagram right over here. So, the sunlight is coming from the left. Sunlight is coming from the left in this sutation. And so if you think about it, what half of the Earth or what part of the Earth is being lit by sunlight? Or what part of the Earth is in daylight? The way I have drawn it over here. Well, the part that is facing the Sun! So, all of this right over here is going to be in daylight. As we rotate, whatever part of the surface of the Earth enters into this yellow part right over here will be in daylight. But let's think about what's happening at different parts of the Earth. Let me draw the Equator which separates our Northern and Southern hemispheres. So, this is the Equator. And then let me go to the Northern Hemisphere. And I wanna show you why when the North Pole is pointed away from the Sun, why this is our winter. So, when we're pointed away from the Sun... or if we go to the Actic Circle. So let me go right over here. Let me go to some point in the Arctic Circle. As it goes, as the Earth rotates every 24 hours, this point on the globe will just rotate around just like that. It will just keep rotating around just like that. My question is: that point at the Arctic Circle that rotates, will it ever see the sunlight? Well, no, it will never see the sunlight. Because the North Pole is tilted away from the Sun. So, what I'm shading here in purple, that part of the Earth that's completely tilted away will never see sunlight. Or at least it won't see sunlight while it's tilted away, while it's in this position in the orbit. It never... I won't say never because once it becames summer, they will be able to see the Sun, so, no sunlight, no day I guess you could say. No daylight. If go to slightly more southern latitudes, so let's say you go over here, maybe that's the latitude of something like New York, San Francisco or somenthing like that. Let's think about what it would see as the Earth rotates every 24 hours. So, this would be daylight, then nighttime, this is now going behind the globe, nighttime, nighttime, daylight, daylight. So, if you just compare this, let me do the daylight in orange, daylight is in orange and nighttime I'll do in this bluish-purplish color. So nighttime over here. If you go to really northern latitudes like the Arctic Circle, they don't get any daylight when we're tilted away from the Sun. If we go to slightly still northern latitudes, but not as northern as the Arctic Circle, it does get daylight, but it gets a lot less daylight. It spends a lot less time in the daylight than in the nigttime. So notice, if you say that this circumference represents the possition over 24 horus, it spends much less time in the daylight than it does in the nighttime. So, because while the Northern Hemisphere is tilted away from the Sun, the latitudes in the Northern Hemisphere are getting less daylight. They're getting less daylight. They're also getting less energy from the Sun, and that's what leads to winter or just being generally colder. And to see what happens in the summer, let's just go to the other side. So, now we're going to the other side of our orbit around the Sun, this is going to be 6 months later. And notice, the actuall direction relative to the rest of the universe has not changed, we're still pointing at that same direction, we still have the 23,4° tilt relativ to I guess being straight up and down. But now, once we're over here, the light from the Sun is going to be coming from the right. Just like that. And now, if on this diagram at least, this is the side of the Earth that is going to be getting the sunlight. Let me draw the Equator again. I'll do my best attempt to draw the Equator. I'll draw the Equator in the same color actually, in that green color. So, this separates the Northern and Southern Hemisphere. And now let's think about the Arctic Circle. So, let's say I'm sitting here in the Arctic Circle. As the day goes on, there's 24 hours going round, I keep rotating around here, but notice, the whole time I'm inside of the sun. I'm getting no nighttime. There is no night in the Arctic circle while we're tilted towards the Sun. And if we still do that in fairly northern latitude, but not as far as in the Arctic Circle, maybe San Francisco or New York, something like that, we go to that latitude, notice how much time we spend in the sun. Maybe we just enter, this is right at sunrise, and then as the day goes on we're in sunlight, sunlight, then we hit sunset, then we hit nighttime, nightime, and then we get sunrise again. And so when you look at the amount of time that something in the Northern Hemisphere spends in the daylight, we see a lot more time in the daylight, when the Northern Hemisphere is tilted towards the Sun. So this is more "day", less night, so it's getting more energy from the Sun. So, when it's tilted towards the Sun, it is getting more energy form the Sun, so things will generally be warmer, and so you are now talking about summer in the Northern Hemisphere. And the arguments for the Southern Hemisphere are identical. You can even play it right over here. When the Northern Hemisphere is tilted away from the Sun, then the Southern Hemisphere tilted towards the Sun. And so for example, the South Pole will have all day light and no night time. Southern latitudes will have more daylight then nighttime. So the South will have summer. So, this is summer in the South, the Southern Hemisphere, and it's winter in the North. And then down here, the Southern Hemisphere is pointed away from the Sun, so, this is winter in the Southern Hemisphere. And you might be saying: Hey Sal, you haven't talked a lot about spring and fall! Well, let's think about it. If we talk about the Northern Hemisphere, this over here we just ... was winter in the Northern Hemisphere and we are going to rotate around the Sun and at some point we're going to get over here. And then, because of this tilt, we aren't pointed away or towards the Sun. We're kind of pointed I guess sideways relative to the direction of the Sun, but this doesn't favour one hemisphere over the other. So, once we're over here, this will actually be the spring now, we're in the spring. Both hemispheres are getting the equal amount of daylight and sunlight. Or for a given latitude above or below the Equator, they're getting the same amount. And the same thing is true over here, when we get to... this is the spring, this is the summer in the Northern Hemisphere, and now this will be the fall in the Northern Hemisphere, and once again, we're tilted in this direction, and so the Northern Hemisphere is not tilted away or towards the Sun, and so both hemispheres are going to get the same amount of radation from the Sun. So, you really see the extremes in the winters and the summers. Now, one thing I do want to make clear, and I started off with just the length of day- and nighttime, because frankly, that's, maybe at least for my brain, a little bit easier to visualise. But that by itself does not account for all of the differences between summer and winter. Another cause, and actually this is probably the biggedst cause, is if you think about the total amount of sun, so let's talk about the Northern Hemisphere winter. And let's say there is a certain amount of sunlight that is reaching the Earht. So there is a certain amount of sunlight that is reaching the Earth. So this is the total amount of sunlight that's reaching the Earth at any point time. You see that much more of that is hitting the Southern Hemisphere than the Northern Hemisphere here. if you imagine it, all of these rays right over here are hitting the Southern Hemisphere. So majority of the rays are hitting the Southern Hemisphere and much fewer are hitting the Northern Hemisphere. So actually a smaller amount of the radiation in period, in even an given period of time, not even talking about the time you're facing the sun, but in any given moment in time more energy is hitting the Southern Hemisphere than the Northern. And the opposite is true when the tilt is then towards the Sun. Now a disproportionate amount of the Sun's energy is hitting the Northern Hemisphere. If you draw a bunch of... just think this is all the energy from the Sun, most of it, all of the rays appear are hitting the Northern Hemisphere, and only these down here are hitting the Southern Hemisphere. And on top of that, what makes is even more extreme, is the actual angle that the... and of course this is to some degree due the fact of where the angle of the Sun in relative to the horizon or where you are on Earth, but even more than that, if you are on... let's say this is the land, and we're talking about the winter in the Northern Hemisphere. So let's say you're talking about... let's say we're up over here, at this northern latitude, and we're just looking at the Sun here. And over here, even when we're closest to the Sun, the Sun is not directly overhead. When we're closest to the Sun, the Sun still is pretty low on the horizon. So, maybe right over here, when we're closest to the Sun in winter, the Sun might be right over here. But if you look at the same latitude in the summer, when it is closest to the Sun, the Sun is more close to being directly overhead. Still won't be directly overhead, because we're still at relatively northern latitude, but the Sun is going to be much higher in sky. And these are all related to each other, it's kind of connected with the idea that more energy is hittig one hemisphere than the other, but also when you have a, I guess you could say a steeper angle from the rays of the Sun with the Earth, it's actually going to be dissipated less by the atmosphere. Let me just make it clear how this is. So in the summer, let's say that that's the land, and let me draw the atmosphere in white. So, all of this area right over here, this is the atmosphere. Obviously, there's not a hard boundary of the atmosphere, but let's just say that this is the densest part of the atmosphere. In the summer when the Sun is higher in the sky, the rays from the Sun are dissipated by less atmosphere. They have to get through this much atmosphere, and they're bounced off and they heat some of that atmosphere before they are absorbed, before they get to the ground. In the winter when the Sun is lower in the sky, so maybe the Sun is out here, when the Sun is lower in the sky relative to this point, you see that the rays of the sunlight have to travel through a lot more atmosphere. So they get dissipated much more before they get to this point on the planet. So all in all, it is the tilt that is causing the changes in the season, but it's causing it for multiple reason. One is when you're tilted towards the Sun, you're getting more absolute hours of daylight, not only you're getting more absolute hours of dayligh, but at any given moment most or more of the Sun's total rays that are hitting the Earth are hitting the Northern Hemisphere as opposite to the Southern Hemisphere, and the stuff that's hitting the places that have summer, it has to go through less atmosphere, so it gets dissipated less.