米兰科维奇假说,岁差与黄赤交角

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 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 Sun is to the left and so the Rays from the Sun are coming in this general direction we've learned that Earth has a certain tilt earth has a tilt and what I mean that it means if you think about the 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 the art 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'm that I've drawn this arrow on it is not straight up and down and right now it is an angle I've it is an angle it is at an angle of it is at an angle of 23 23 point four degrees with the vertical width 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 is going through the atmosphere has to go through less atmosphere and do the things in the northern hemisphere are getting more daylight and when you go into the when you when the earth is on the other side of the Sun and it 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 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 very and even the boundaries for this varying will are different than for the past million years and they will be for the next million years but it varies roughly between twenty-two point one degrees and twenty-four point five degrees and just to make it clear that it's not going it's not wobbling back and forth like this and just to visualize twenty-two point one versus 24 point five it's not a huge difference so this is twenty-three point four and I'm not measuring exactly maybe pointing in this direction may be pointing may be pointing in that maybe twenty two point one would look something like that in fact I have exaggerated it and maybe twenty four point five would go 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 a significant impact on what the climate is like or what the seasons are like especially in terms of how the how how much of a chance different parts of our of our planet have a chance to freeze over and not freeze over and all the rest or how much sunlight they get in 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 forty-one thousand years and right now at a tilt of 23 point four degrees where someplace right smack in between and we we we think the last minimum the the or started the last maximum was that was it 8700 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 will be at at eleven thousand in the year eleven thousand eight hundred 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 fancy word for tilt is sometimes given is Oh big obliquity oblique WA T but this is really just a fancy word for tilt this this changing of the obliquity or changing of the tilt is one of these chain 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 eight ice ages when they when they act it together with each other over certain cycles and broadly this entire class of cycles are called Milankovitch cycles Milankovitch use the 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 Milankovitch Milankovitch cycles and changes in the tilt or the obliquity are just one of the possible factors playing into Milankovitch cycles and what i want to do in this video in the next view is talk about all of the different factors or at least let's 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 i've precession precession 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 the top is spinning in this direction and oblique OD tells you essentially how much it's wobbling well actually we think of it this way imagine a wobbling top so it's rotating like this it's tilted and then it's also it is also it is 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 that arrow itself would be rotating so what you the best way to think about it is a wobbling top if you think if it's after some point of time this thing would wobble so it would look like 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 I mean now maybe the arrow is pointing a little bit out of the page and then you wait a few more seconds and it's pointing in this direction there's pointing into the page and so this whole time the oblique he isn't changing the OBE liquidy you can kind of view it as how far 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 as the precession as where we are in the wobble 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 just as it takes a long time for the for the inclination to change from a minimum value to a maximum value in back it takes a huge amount of time for Earth's precession to change in a significant way so for this top two kind of four if you imagine to this 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 gonna 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 unless let me 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 let's say it is tilted towards the Sun right now so it is tilted towards the Sun and so if in the northern hemisphere and I'm assuming this arrows 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 you saying you still have let me do that in blue you still have the same the same direction of tilt we're still pointing to the same part of the universe we still have the same Northstar 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 if you had no procession when you get back to this point over here we'd be tilting in the exact same direction you mean if you're if you're if your oblique WA T or your change 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 this let me draw 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 oh blick wa t is how much we're going from from vertical and so if we had no precession we would be exactly pointing in that same direction every year now if we with just precession alone what happens is every year this this arrow is slowly tracing out a circle 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 it is now pointing in this direction 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 the the summer in the northern hemisphere well it won't because we're now not pointing directly door were 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 we would have been most inclined the Sun may be over here may be over here and it would take a many many many actual thousands of years for it to get to be too forth for the procession to change this much but then over here this is where when at this point in that year when when we would be pointed most towards the Sun so what the the real effect of precession is 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 the slowly tracing out this circle with I guess you could say with it's pole what it's doing is is it's making it it's making it tilt towards the Sun or away from the Sun a little bit earlier each year a little bit earlier I know it's hard to visualize but you can even take a top out and have I don't know have a basketball as the Sun and if you play with it you'll see how that works and the precession is another one of those factors that affect that that play into I should say the Milankovitch cycles and what we'll see is when you combine precession when you combine or I should say change in precession when you had combined that with changes in tilt and you combine that with changes in actual a house circular or how elliptical the actual orbit is and how that changes then you might have a a respectable way of explaining what or some of explaining why earth is entered into these climactic cycles over many tens of thousands of years