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Milankovitch cycles precession and obliquity

How changes in Earth's rotation can effect Earth's seasons and climate. Created by Sal Khan.

Video transcript

We've learned in previous videos that relative to the orbital plane around the sun or the plane of Earth's orbit around the sun, the Earth has a certain tilt. So let me draw the Earth's tilt relative to that orbital plane right over here. So this if this is the orbital plane right over here so we're looking right directly sideways on this orbital plane that I've drawn in orange. And maybe at the point in Earth's orbit right now, maybe the sun is to the left, and so the rays from the sun are coming in this general direction. We've learned that the Earth as a certain tilt, and when I mean that it means if you think about the axis around which it's rotating it's not straight up from the orbital plane, it is at an angle. And let me draw that. So if I were to draw an arrow that's coming out of the North Pole it would look like that. And maybe I'll draw an arrow coming out of the South Pole. And the Earth is rotating in that direction right over here, and you notice this axis that I've drawn this arrow on, it is not straight up and down. And right now it is at an angle of 23.4 degrees with the vertical, with being straight up and down. And we've learned how this is what is the primary cause of our seasons in that when the Northern Hemisphere is pointed towards the sun it's getting a disproportionate amount of the solar radiation. Whatever's going through the atmosphere has to go through less atmosphere and the things in the Northern Hemisphere are getting more daylight. And when the Earth is on the other side of the sun and the Northern Hemisphere is pointed away from the sun then the opposite is going to happen. And the reverse is true for the Southern Hemisphere. But in that video when we talk about how tilt can affect the seasons, I also kind of hinted a little bit that this is the current tilt right now, and over long periods of time that this tilt will change. And in particular, it will vary, and even the boundaries for this varying are different for the past million years than they will be for the next million years, but it varies roughly between 22.1 degrees and 24.5 degrees. And just to make it clear that it's not wobbling back and forth like this, and just to visualize 22.1 versus 24.5, it's not a huge difference. So if this is 23.4-- and I'm not measuring exactly-- maybe pointing in this direction, maybe 22.1 would look something like that. In fact, I've exaggerated it. And maybe 24.5 would look something like that. And so it's not a huge difference, but it is enough of a difference so we believe to actually have a significant impact on what the climate is like or what the seasons are like, especially in terms of how much of a chance different parts of our planet have a chance to freeze over or not freeze over and all the rest, or how much sunlight they get and all the rest. So it has some impact, but I want to make it clear that it takes a long period of time. That it actually takes 41,000 years to go from a minimum tilt to a maximum tilt and then back to a minimum tilt. 41,000 years. And right now at a tilt of 23.4 degrees, we're someplace right smack in between. And we think the last maximum was at 8,700 BCE, before the common era, or you could say before Christ. And that the next minimum, when our tilt has been minimized, the next time our tilt will be minimized will be in the year 11,800. So this isn't something that's happening overnight, but it is something that could affect our climate over long periods of time. And this is just one factor, and sometimes this changing of the tilt, a fancier word for tilt is sometimes given, is obliquity, but this is really just a fancy word for tilt. This changing of the obliquity-- or the changing of the tilt-- is one of these changes in Earth's rotation or Earth's orbit around the sun that might have long-term cycles or effects on Earth's climate, and maybe they do help cause certain ice ages when they act together with each other over certain cycles. And broadly, this entire class of cycles are called Milankovitch cycles. Milankovitch, he was a Serbian scientist, who's the guy who theorized that these changes in Earth's orbit might be responsible for long-term climate change or maybe some cycles where we enter ice ages and get out of ice ages or we have more extreme or less extreme weather. So these are Milankovitch cycles. And changes in the tilt, or the elbow obliquity, are just one of the possible factors playing into Milankovitch cycles. And what I want to do in this video and in the next video is talk about all of the different factors, or at least summarize all of the different factors. Now, another one. This one is pretty intuitive for me that this tilt can change. One that's a little bit less intuitive when you first think about it is something called precession. And the idea behind precession, I guess the best analogy I can think of, is if you imagine a top, or maybe you could imagine Earth as a top right over here. The top is spinning, the top is spinning in this direction, and obliquity tells you, essentially, how much it's wobbling. Actually, let me think of it this way. Imagine a wobbling top. So it's rotating like this, it's tilted, and then it is also if you imagine that this was a pole up here that's coming out of the pole, that this was actually a physical arrow, that that arrow itself would be rotating. So the best way to think about it is a wobbling top. If you think after some point of time this thing would wobble so it would look like this. So now the arrow is pointing that way. And if you wait a few more seconds now maybe the arrow is pointing a little bit out of the page. And then you wait a few more seconds then it's pointing in this direction, it's pointing into the page. And so this whole time, the obliquity isn't changing. The obliquity you can view it as how far is that wobble. You could imagine how far from vertical is that wobble, and no matter where we are in that rotation it hasn't changed, and you could imagine the precession as where we are in the wobble. And I want to-- this is a little bit hard to visualize, and hopefully, as we think about it in different ways and I draw different diagrams it'll become a little bit clearer-- but I want to make it clear. Just as it takes a long time for the inclination to change from a minimum value to a maximum value and back, it takes a huge amount of time for Earth's precession to change in a significant way. So for this top to-- if you imagined this arrow popping out, for this arrow to actually trace out an entire loop, it takes 26,000 years. So 26,000 years to have an entire cycle of precession. Now, what I want to do is think about given that this precession is occurring, I want to think about how that would affect our seasons, or how it would actually affect how we think about the year or the calendar. So let's draw the orbit of Earth around the sun. So here is my sun right over here. And here is the orbit of Earth. And I'm not going to think too much-- I'm going to assume that it's almost circular for the sake of this video. In future videos, we'll talk about how the eccentricity-- or how elliptical the orbit is-- can also affect the Malinkovitch cycles or play into the Malinkovitch cycles. But let's just draw the orbit of Earth around the sun over here. And so you could imagine this is, at one point in time, this is the Earth. Let's say it is tilted towards the sun right now. And so is in the Northern Hemisphere-- and I'm assuming this arrow is coming out of the North Pole-- this would be the summer in the Northern Hemisphere. And then if you had no precession, absolutely no precession, when you go to this time of year you still have the same direction of tilt. Let me do that in blue. You still have the same direction of tilt. We're still pointing to the same part of the universe. We still have the same North Star. You go to this time, we're still tilting in the same direction relative to the universe, but we're not tilting away from the sun. And now this would be the winter in the Northern Hemisphere. And we'd keep going around. And if you had no precession, when you get back to this point over here, we'd be tilting in the exact same direction. If your obliquity or if your tilt changed a little bit, you might move up or down, away, or towards the sun a little bit, but this is all assuming no precession. Now, I'm going to think about what happens if you do have precession. So what's happening with precession is when you go around one time around the sun, by the time you get to this point again you're not pointing at exactly the same direction. You are now pointing a little bit further so this arrow-- let me draw it a little bit bigger. So this is the Earth and this is that arrow. And this is hard to visualize or at least it's hard for me to visualize. Well, once you get it, it's easier to visualize, but the first time I tried to understand it, it was hard for me to understand how precession was different than obliquity or different than tilt. Obliquity is how much we're going from vertical. And so if we had no precession, we would be exactly pointing in that same direction every year. Now, with just precession alone what happens is every year this arrow is slowly tracing out a circle that goes like this. So I'm going to exaggerate how much it's happening just so that you can visualize it. So maybe after several years that arrow is not-- when you're at that same point relative to the sun, that same point in the solar system, that arrow is no longer pointing in that direction. It is now traced out a little bit of that circle. So it is now pointing in this direction. So if it is now pointing in this direction, will that same point in the solar system, that same point relative to the sun, that same exact point in the orbit, will it still be the summer in the Northern Hemisphere? Well, it won't because we're now not pointing directly or we're not most inclined to the sun at that point. Now, we would have been most inclined to the sun a little bit earlier in the year or a little bit earlier in the orbit. So we would have been most inclined to the sun maybe over here. And it would take many, many, many actual thousands of years for the precession to change this much, but then over here this is where when at this point in that year when we would be pointed most towards the sun. So what the real effect of precession is doing to our seasons and doing to what our sense of what our year is, is that every year relative to our orbit on Earth, because Earth is kind of a top that's slowly circling, that's slowly tracing out this circle with, I guess you could say, with its pole. What it's doing is it's making it tilt towards the sun or away from the sun a little bit earlier each year. I know it's hard to visualize, but you could even take a top out and have a basketball as the sun, and if you play with it, you'll see how that works. And precession is another one of those factors that play into, I should say, the Malinkovitch cycles. And what we'll see is when you combine precession, when you combine-- or I should say change in precession, when you combine that with changes in tilt, and you combine that with changes in actual how circular or how elliptical the actual orbit is and how that changes, then you might have a respectable way of explaining, or some of explaining, why Earth is entered into these climactic cycles over many tens of thousands of years.