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You don't think that AD stands for after death. It stands for Anno Domini, the year of the Lord or the year of our Lord. And CE stands for Common Era. But this and this are referring to essentially the same count after the birth of Christ, or this theoretical birth of Christ, which we don't really know when it actually... | Understanding calendar notation Cosmology & Astronomy Khan Academy.mp3 |
But this and this are referring to essentially the same count after the birth of Christ, or this theoretical birth of Christ, which we don't really know when it actually happened. It probably did not happen at the beginning of 1 AD or 1 CE. And these two things both also refer to the same direction in the timeline. One... | Understanding calendar notation Cosmology & Astronomy Khan Academy.mp3 |
One last thing I want to point out is that there is no year zero. So if you take either of these naming schemes, you have, so let's go very close to the year one. So there's just some point, this theoretical birth of Christ, which was probably not the actual birth of Christ, but at that theoretical point, right at that... | Understanding calendar notation Cosmology & Astronomy Khan Academy.mp3 |
All of a sudden now, you are on January 1 of 1 AD or CE, depending on how you want to refer to it. And the year before that was 1 BC or BCE, depending on how you want to refer to it. So there is no year zero in this scheme. And then the last thing I want to emphasize, and it might be obvious to you, is the larger a num... | Understanding calendar notation Cosmology & Astronomy Khan Academy.mp3 |
And then the last thing I want to emphasize, and it might be obvious to you, is the larger a number you have here, the further back you're going in time. Because this is saying how many years before this theoretical birth of Christ. And obviously, larger numbers you have here, this is the further you're going off into ... | Understanding calendar notation Cosmology & Astronomy Khan Academy.mp3 |
And if you wanted to figure out how many years passed between Plato's birth and Columbus sailing across the Atlantic to find the New World, you would say, well, look, it took 428 years to go from Plato's birth to this theoretical birth of Christ. And then you have another 1,492 years to wait until Columbus gets his shi... | Understanding calendar notation Cosmology & Astronomy Khan Academy.mp3 |
And when we think of farming, we imagine a farmer planting seeds and later harvesting the crops, or maybe having cattle that they can allow to graze and then using that cattle for either meat or milk or wool. But there's actually a different type of farming that predates this association with kind of, I guess what we c... | Firestick farming Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3 |
And we believe that it started with the original inhabitants of Australia. And what they did is, and this is why we call it farming, because if you think about farming in the most general sense, it's really humans using technology to manipulate their environment so it becomes more suitable for humans, so it becomes mor... | Firestick farming Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3 |
Fire stick farming. And I think you can already imagine what it might involve. It involves using fire, which is really a form of technology, or it can be a form of technology, using fire to make the environment more suitable for human activity. And so what the original Australians did, the indigenous Australians, or so... | Firestick farming Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3 |
And so what the original Australians did, the indigenous Australians, or sometimes referred to as the Aboriginal Australians, Aboriginal Australians, and if you're wondering where the word Aboriginal comes from, you might recognize some parts of it. Original, you know what that means, the first things, the things that ... | Firestick farming Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3 |
So this is literally from the beginning. So when you say Aboriginal Australians, you're really kind of saying the Australians that were there from the beginning. And so what they would do is, is that we believe if you go back 50 or 60,000 years, before the first Aboriginal Australians settled Australia, Australia had m... | Firestick farming Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3 |
It still has forest. This is a modern picture, obviously, of an Australian forest. But what they did is that they set up controlled burns. And what these controlled burns did is that they cleared away a lot of the forest, they cleared away a lot of the brush that's at the bottom of, you know, that's over here, and it m... | Firestick farming Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3 |
And what these controlled burns did is that they cleared away a lot of the forest, they cleared away a lot of the brush that's at the bottom of, you know, that's over here, and it made it much more suitable for grassland to develop. And the reason why they liked grassland, so let's make a little cycle here of what they... | Firestick farming Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3 |
And then once you have grassland, that made the environment more suitable for animals that the original human settlers could essentially live off of, that they could hunt, that they could potentially eat their meat. And so, for example, things like kangaroos, and these supported the human population, which obviously wo... | Firestick farming Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3 |
Someone provides a controlled burn. And they were actually pretty scientific about how they did it. They wouldn't just go at the end of summer when everything was hot and ready to just blow up and then start a fire that they couldn't control. They would often do these in seasons knowing that it had a certain level of m... | Firestick farming Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3 |
They would often do these in seasons knowing that it had a certain level of moisture in the air, it wasn't too hot. And to a large degree, by doing these controlled burns, not only did it provide an environment, kind of do this fire stick farming, not only did it provide an environment that was suitable for things like... | Firestick farming Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3 |
And there's some reason to believe that what the original Australians did, on some level, was more nuanced and more fine-tuned than even what we do in a modern sense in controlled burns. So these controlled fires also prevented major uncontrollable fires. Because what happens is if you don't have these controlled fires... | Firestick farming Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3 |
You have stuff building up. And then when the fires do occur, they're not going to occur, or they're less likely, the uncontrolled fires are less likely to be started during the winter when the air is cool or when there might be some moisture. They're more likely to occur in the dry season. So you have all this stuff b... | Firestick farming Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3 |
So you have all this stuff build up, and then when the fire does happen, it happens in the driest season. And then when it happens with all of the stuff build up in the dry season, it just becomes uncontrollable. One of the byproducts, or actually there's several byproducts, of this fire stick farming, we believe, is a... | Firestick farming Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3 |
And even when the first European settlers came in the late 1700s, they were kind of surprised when they went into what is now Sydney Harbor, and they said, wow, look at all the grassland here. It almost looks like park space. And then they would let their sheep graze there, and they were surprised, because they had dri... | Firestick farming Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3 |
And it was because the original Australians were actually controlling that forest growth to make it more inhabitable for things like kangaroos. And then when the English settlers came, they started to have their sheep graze in those grasslands. And it also is responsible for the disappearance, we think, of many major, ... | Firestick farming Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3 |
So really large animals that inhabited Australia for really millions of years until humans showed up. And this is one of them. It's just neat to look at them. This is called Driptodon optimum, or another way to think of it, the giant wombat. And there's fossils of the giant wombat around 40,000, 50,000 years ago. But t... | Firestick farming Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3 |
This is called Driptodon optimum, or another way to think of it, the giant wombat. And there's fossils of the giant wombat around 40,000, 50,000 years ago. But they disappeared with humans showing up. And there's multiple ways that you could think about why they disappeared. They might have, and this is probably the ca... | Firestick farming Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3 |
And there's multiple ways that you could think about why they disappeared. They might have, and this is probably the case, they might have been more dependent on the forest habitat. Or this was a more favorable habitat for them than the grasslands, maybe because they ate leaves that were high up. Or another thing is on... | Firestick farming Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3 |
And what's neat about them is, one, because they're so large and so bright, you can see them really, really far away. And what's even neater about them is that they're variable, that they pulsate. And because their pulsations are related to their actual luminosity, you know if you see a Cepheid variable star in some di... | Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3 |
And so if you know its actual luminosity, and then you know, obviously, its apparent luminosity, you know how much it's gotten dim. And the more dim it's gotten from its actual state, you know the farther away it is. So that's the real value of them. What I want to do in this video is to try to explain why they're vari... | Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3 |
What I want to do in this video is to try to explain why they're variable, why they pulsate. And to do that, what we're going to think about is doubly and singly ionized helium. And just to review, helium, so neutral helium, let me draw neutral helium. Neutral helium's got two protons, two neutrons, and then two electr... | Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3 |
Neutral helium's got two protons, two neutrons, and then two electrons. And obviously, this is not drawn to scale. So this is neutral helium right over here. Now, if you singly ionize helium, you knock off one of these electrons. And these type of things happen in stars, when you have a lot of heat, easier to ionize th... | Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3 |
Now, if you singly ionize helium, you knock off one of these electrons. And these type of things happen in stars, when you have a lot of heat, easier to ionize things. So singly ionized helium will look like this. It'll have the same nucleus, two protons, two neutrons. One of the electrons gets knocked off, so now you ... | Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3 |
It'll have the same nucleus, two protons, two neutrons. One of the electrons gets knocked off, so now you only have one electron. And now you have a net positive charge. So here, let me do this in a different color. This helium now has a net charge. We could write 1 plus here, but if you just write a plus, you implicit... | Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3 |
So here, let me do this in a different color. This helium now has a net charge. We could write 1 plus here, but if you just write a plus, you implicitly mean a positive charge of 1. Now, you can also doubly ionize helium if the environment is hot enough. You can doubly ionize helium. And doubly ionizing helium is essen... | Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3 |
Now, you can also doubly ionize helium if the environment is hot enough. You can doubly ionize helium. And doubly ionizing helium is essentially knocking off both of the electrons. So then it's really just a helium nucleus like this. This right here is doubly ionized helium. Now, I just said, in order to do this, you h... | Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3 |
So then it's really just a helium nucleus like this. This right here is doubly ionized helium. Now, I just said, in order to do this, you have to have a hotter environment. There has to be a hotter environment in order to be able to knock off both. This electron really doesn't want to leave. To take an electron off of ... | Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3 |
There has to be a hotter environment in order to be able to knock off both. This electron really doesn't want to leave. To take an electron off of something that's already positive is difficult. You have to have a lot of, really, pressure and temperature. This is cooler. And this is all relative. We're talking about th... | Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3 |
You have to have a lot of, really, pressure and temperature. This is cooler. And this is all relative. We're talking about the insides of stars. So this is a hotter part of the star versus a cooler part of the star, I guess, is the way you think about it. It's a very hot environment by our traditional everyday standard... | Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3 |
We're talking about the insides of stars. So this is a hotter part of the star versus a cooler part of the star, I guess, is the way you think about it. It's a very hot environment by our traditional everyday standards. Now, the other thing about the doubly ionized helium is that it is more opaque, which means it doesn... | Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3 |
Now, the other thing about the doubly ionized helium is that it is more opaque, which means it doesn't allow light to go through it. It actually absorbs light. It is more opaque. It absorbs light. Or another way, it absorbs that light energy. That energy will make it even hotter. So that's just something to think about... | Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3 |
It absorbs light. Or another way, it absorbs that light energy. That energy will make it even hotter. So that's just something to think about. Now, the singly ionized helium is more transparent. It allows the light to pass through it. So it doesn't get heated as much by photons that are kind of going near it or through... | Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3 |
So that's just something to think about. Now, the singly ionized helium is more transparent. It allows the light to pass through it. So it doesn't get heated as much by photons that are kind of going near it or through it or whatever. It allows them to go through it. Here, the photons are going to actually heat up the ... | Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3 |
So it doesn't get heated as much by photons that are kind of going near it or through it or whatever. It allows them to go through it. Here, the photons are going to actually heat up the ion. So let's think about how this might cause a Cepheid variable to pulsate. So assuming that Cepheid variables have large enough qu... | Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3 |
So let's think about how this might cause a Cepheid variable to pulsate. So assuming that Cepheid variables have large enough quantities, I should say, of these ions, we can imagine that when a Cepheid variable is dim. So let me draw a dim Cepheid variable. So I'll draw that like I'll draw this in a dim color. So this ... | Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3 |
So I'll draw that like I'll draw this in a dim color. So this is a dim Cepheid variable right here. In its dim state, just like this, you have a lot of the doubly ionized helium in the star, at least kind of the outer surface of the star. And so this does not allow a lot of light to pass through. So this is the dim par... | Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3 |
And so this does not allow a lot of light to pass through. So this is the dim part of the pulsation of the Cepheid variable. Now, because this doubly ionized helium is opaque, it is absorbing the light. It is getting heated. It is getting heated. And because it's getting heated, it'll cause the star to expand. So becau... | Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3 |
It is getting heated. It is getting heated. And because it's getting heated, it'll cause the star to expand. So because it's getting heated, it'll become more energetic, and the star will actually expand. Now, as the star expands, because this doubly ionized helium is getting heated, what's going to happen? The further... | Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3 |
So because it's getting heated, it'll become more energetic, and the star will actually expand. Now, as the star expands, because this doubly ionized helium is getting heated, what's going to happen? The further away you are from the core of the star, the cooler it gets. So this expanded because it was getting heated, ... | Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3 |
So this expanded because it was getting heated, but then because it expanded, the outer layers of the star become cooler. And since they're cooler, helium won't be doubly ionized anymore. It'll get an electron from each helium atom. It can now get an electron from the plasma, I guess we can say, to become singly ionize... | Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3 |
It can now get an electron from the plasma, I guess we can say, to become singly ionized helium. So now we have singly ionized helium. And now the star is going to be more transparent. It's going to allow more light to pass through it. So now this is the bright part of the pulsation. It's going to allow more light thro... | Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3 |
It's going to allow more light to pass through it. So now this is the bright part of the pulsation. It's going to allow more light through. So now it is bright. The star is bright. But what's happening now? Because the light is no longer, or it's not being absorbed as well by the helium when it was a doubly ionized hel... | Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3 |
So now it is bright. The star is bright. But what's happening now? Because the light is no longer, or it's not being absorbed as well by the helium when it was a doubly ionized helium, now it's letting most of the light, or a lot more of the light, get through. It's not going to get heated as much. And so it won't have... | Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3 |
Because the light is no longer, or it's not being absorbed as well by the helium when it was a doubly ionized helium, now it's letting most of the light, or a lot more of the light, get through. It's not going to get heated as much. And so it won't have the kinetic energy to kind of keep pushing out, to keep moving out... | Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3 |
And so it'll collapse back into the star. And so then this will cool down and collapse back in. And when it collapses back in, what's going to happen? When it collapses back in, when these helium atoms get closer to the center of the star, to the core of the star, they're going to be heated again, because they're close... | Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3 |
When it collapses back in, when these helium atoms get closer to the center of the star, to the core of the star, they're going to be heated again, because they're closer now to the core. And when they get heated, they're going to become doubly ionized. So then we have doubly ionized helium again. And then the cycle wi... | Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3 |
And then the cycle will go again. It is now opaque. It will now absorb more energy. That'll cause it to have more kinetic energy to expand. Once it expands, it'll get cool again, and transparent, and bright. And so this is the current best theory of why Cepheid variable stars are variable to begin with. It's this whole... | Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3 |
I said that you had this state 300,000 years. So we talk about the Big Bang happening 13.7 billion years ago. And then I talk about this state of affairs where we're maybe 30 million light years away from the edge of the observable universe, the current observable universe. And I said that this was about 300,000 years ... | Radius of observable universe (correction) Cosmology & Astronomy Khan Academy.mp3 |
And I said that this was about 300,000 years after the Big Bang. That's what I talked about in the last video. That was our starting point when the photon started leaving that point, and obviously the universe expanding. The photon, it kind of traversed more and more, but still had more and more to travel as the univer... | Radius of observable universe (correction) Cosmology & Astronomy Khan Academy.mp3 |
The photon, it kind of traversed more and more, but still had more and more to travel as the universe expanded, as all of space expanded. But this is 300,000 years after the Big Bang. Now my brain, because I was kind of not thinking hard enough about it, I said, hey, this was 13.4 billion years ago. That's what I incor... | Radius of observable universe (correction) Cosmology & Astronomy Khan Academy.mp3 |
That's what I incorrectly said in the last video. I said that this is 13.4 billion years ago. That's what I said in the last video, and that is wrong. Because if this was 13.4 billion years ago, this would have been 300 million years after the Big Bang. We were talking about only 300,000 years after the Big Bang, so it... | Radius of observable universe (correction) Cosmology & Astronomy Khan Academy.mp3 |
Because if this was 13.4 billion years ago, this would have been 300 million years after the Big Bang. We were talking about only 300,000 years after the Big Bang, so it wouldn't have taken that many decimal places off of something in the billions. The correct answer is this would have been only a little less than 13.7... | Radius of observable universe (correction) Cosmology & Astronomy Khan Academy.mp3 |
It actually wouldn't have even made the significant digits. This is still approximately 13.7 billion years ago. So I wanted to just make that correction. It was a slight error. I shouldn't have viewed this as 0.3 billion years. This is only 0.3 million years. It doesn't even basically change the precision on this numbe... | Radius of observable universe (correction) Cosmology & Astronomy Khan Academy.mp3 |
So let me draw the Earth's tilt relative to that orbital plane right over here. So if this is the orbital plane right over here, so we're looking right directly sideways on this orbital plane, right sideways along this orbital plane that I've drawn in orange, and maybe at the point in Earth's orbit right now, maybe the... | Milankovitch cycles precession and obliquity Cosmology & Astronomy Khan Academy.mp3 |
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 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 righ... | Milankovitch cycles precession and obliquity Cosmology & Astronomy Khan Academy.mp3 |
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 t... | Milankovitch cycles precession and obliquity Cosmology & Astronomy Khan Academy.mp3 |
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 pointing in that, maybe 22.1 would look something like that. In fact, I've exaggerated... | Milankovitch cycles precession and obliquity Cosmology & Astronomy Khan Academy.mp3 |
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 f... | Milankovitch cycles precession and obliquity Cosmology & Astronomy Khan Academy.mp3 |
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... | Milankovitch cycles precession and obliquity Cosmology & Astronomy Khan Academy.mp3 |
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 at 11,800. So this isn't something that's happening overnight, but it is something that could affect our c... | Milankovitch cycles precession and obliquity Cosmology & Astronomy Khan Academy.mp3 |
But this is really just a fancy word for tilt. This changing of the obliquity, or 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 ... | Milankovitch cycles precession and obliquity Cosmology & Astronomy Khan Academy.mp3 |
Milankovitch, he was a Serbian scientist who was 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 chang... | Milankovitch cycles precession and obliquity Cosmology & Astronomy Khan Academy.mp3 |
And what I want to do in this video and the next few 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 p... | Milankovitch cycles precession and obliquity Cosmology & Astronomy Khan Academy.mp3 |
And the idea behind precession, I guess the best analogy I can think of, is if you imagine a top, or maybe you can imagine Earth as a top right over here. The top is spinning in this direction. And obliquity tells you essentially how much it's wobbling. Well, actually let me think of it this way. Imagine a wobbling top... | Milankovitch cycles precession and obliquity Cosmology & Astronomy Khan Academy.mp3 |
Well, actually let me think of it this way. Imagine a wobbling top. So it's rotating like this, it's tilted, and then it's also, if you imagine that this was a pole up here that's coming out of the pole, if this was actually a physical arrow, that that arrow itself would be rotating. So the best way to think about it i... | Milankovitch cycles precession and obliquity Cosmology & Astronomy Khan Academy.mp3 |
So the best way to think about it is a wobbling top. 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 poi... | Milankovitch cycles precession and obliquity Cosmology & Astronomy Khan Academy.mp3 |
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, then it's pointing into the page. And so this whole time the obliquity isn't changing. The obliquity you can kind of view it as how far is that wo... | Milankovitch cycles precession and obliquity Cosmology & Astronomy Khan Academy.mp3 |
The obliquity you can kind of view it as how far is that wobble? You can imagine how far from vertical is that wobble. And no matter where we are in that rotation, it hasn't changed. And you can imagine it as a procession as where we are in the wobble. And I want to, this is a little bit hard to visualize, and hopefull... | Milankovitch cycles precession and obliquity Cosmology & Astronomy Khan Academy.mp3 |
And you can imagine it as a procession 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 make it a little bit clearer. But I want to make it clear, just as it takes a long time for the inclinati... | Milankovitch cycles precession and obliquity Cosmology & Astronomy Khan Academy.mp3 |
So for this top to kind of, 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 affec... | Milankovitch cycles precession and obliquity Cosmology & Astronomy Khan Academy.mp3 |
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 al... | Milankovitch cycles precession and obliquity Cosmology & Astronomy Khan Academy.mp3 |
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 Milankovitch cycles or play into the Milankovitch cycles. But let's just draw the orbit of Earth around the sun over here. And so you can i... | Milankovitch cycles precession and obliquity Cosmology & Astronomy Khan Academy.mp3 |
And so you can imagine this is at one point in time. This is the Earth. Let's say it is tilted towards the sun right now. So 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 prec... | Milankovitch cycles precession and obliquity Cosmology & Astronomy Khan Academy.mp3 |
So 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 hav... | Milankovitch cycles precession and obliquity Cosmology & Astronomy Khan Academy.mp3 |
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. At 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... | Milankovitch cycles precession and obliquity Cosmology & Astronomy Khan Academy.mp3 |
At 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. If you had no precession, when you get back to this point over here, we'd be tilting in the exact same direc... | Milankovitch cycles precession and obliquity Cosmology & Astronomy Khan Academy.mp3 |
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 ... | Milankovitch cycles precession and obliquity Cosmology & Astronomy Khan Academy.mp3 |
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're now pointing a little bit further. So this arrow, let me draw it a li... | Milankovitch cycles precession and obliquity Cosmology & Astronomy Khan Academy.mp3 |
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 dif... | Milankovitch cycles precession and obliquity Cosmology & Astronomy Khan Academy.mp3 |
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 di... | Milankovitch cycles precession and obliquity Cosmology & Astronomy Khan Academy.mp3 |
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... | Milankovitch cycles precession and obliquity Cosmology & Astronomy Khan Academy.mp3 |
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 direct... | Milankovitch cycles precession and obliquity Cosmology & Astronomy Khan Academy.mp3 |
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 po... | Milankovitch cycles precession and obliquity Cosmology & Astronomy Khan Academy.mp3 |
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, at this point in that year, when we would be pointed most towards the sun. So what the real effect of precession is doin... | Milankovitch cycles precession and obliquity Cosmology & Astronomy Khan Academy.mp3 |
And when it is closer to the sun, so let's say that this is the time of the orbit when it's closer to the sun, this is the perihelion. And when it's furthest from the sun, and I'm exaggerating the difference, this is aphelion. This is the aphelion in our orbit when we are furthest from the sun. Maybe our orbit looks so... | Are southern hemisphere seasons more severe Cosmology & Astronomy Khan Academy.mp3 |
Maybe our orbit looks something like this. And what I point out in the first video where we discussed this is that this is not the cause of the seasons. Even though we are 3% closer right now, the way our orbit is set up, and we'll see in future videos that the difference or the eccentricity or how elliptical the orbit... | Are southern hemisphere seasons more severe Cosmology & Astronomy Khan Academy.mp3 |
But right now, when we are closest to the sun, we are 3% closer than when we are furthest from the sun. So 3% closer than at aphelion. And we point out in the first video when we discussed this, that this is not the cause of the seasons. And in particular, perihelion, when we are closest to the sun, when we actually ha... | Are southern hemisphere seasons more severe Cosmology & Astronomy Khan Academy.mp3 |
And in particular, perihelion, when we are closest to the sun, when we actually have the most radiation from the sun, that's actually when we have the northern hemisphere winter. So this occurs right over here. This occurs in January. And aphelion occurs in July. Now, based on this, this might lead to an interesting qu... | Are southern hemisphere seasons more severe Cosmology & Astronomy Khan Academy.mp3 |
And aphelion occurs in July. Now, based on this, this might lead to an interesting question. Because, so let's think about January when we're at perihelion, and let's think about July when we're at aphelion. And let me draw a quick globe right over here. And let's make that the equator. And I'll draw it in both situati... | Are southern hemisphere seasons more severe Cosmology & Astronomy Khan Academy.mp3 |
And let me draw a quick globe right over here. And let's make that the equator. And I'll draw it in both situations. So January is obviously when we have the northern hemisphere winter. So I'll paint it in blue right over here. It is winter. And July is when we have the northern hemisphere summer or the southern hemisp... | Are southern hemisphere seasons more severe Cosmology & Astronomy Khan Academy.mp3 |
So January is obviously when we have the northern hemisphere winter. So I'll paint it in blue right over here. It is winter. And July is when we have the northern hemisphere summer or the southern hemisphere winter. So then we have winter during July in the southern hemisphere. And let me put summer in a more summery c... | Are southern hemisphere seasons more severe Cosmology & Astronomy Khan Academy.mp3 |
And July is when we have the northern hemisphere summer or the southern hemisphere winter. So then we have winter during July in the southern hemisphere. And let me put summer in a more summery color. I guess that orange is a pretty good color. That's not orange. Here's orange. All right. | Are southern hemisphere seasons more severe Cosmology & Astronomy Khan Academy.mp3 |
I guess that orange is a pretty good color. That's not orange. Here's orange. All right. That's orange. And that's orange. So these are summer. | Are southern hemisphere seasons more severe Cosmology & Astronomy Khan Academy.mp3 |
All right. That's orange. And that's orange. So these are summer. So that's the summer in the southern hemisphere, which occurs during the winter in the northern hemisphere, and vice versa. Summer in the northern hemisphere occurs during winter in the southern hemisphere. And so the question might be rising in your hea... | Are southern hemisphere seasons more severe Cosmology & Astronomy Khan Academy.mp3 |
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