地球的倾角跟季节有何联系

in the last video we talked about how seasons on earth are not caused by how close earth is to the Sun in its orbit and we also hint at the fact that it's actually caused by the tilt of the earth and so in this video I want to show you how the tilt of the earth causes the seasons to happen so let's draw so I'm gonna 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 let's we could 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 look something like that let me draw it almost it is almost circular so I'll draw it is something that's pretty close to a circle right over here and I'm going to draw Earth at different points in its orbit and I'm going to try to depict it's in the tilt of its rotational axis and obviously this is not drawn anywhere near close to scale or there's 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 and let me and we'll 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 we are up and out so if I wanted to put for some perspective on an arrow it would be up and actually be more like 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 and if you were to go straight out out of the South Pole you would go below that circle right over there and if I wanted to draw the same position but if we're looking sideways along the plane the orbital plane of or the plane of Earth's of Earth's orbit so if we're looking at from that direction so let me do it this way if we're looking at directly sideways directly sideways this is the Sun right over here this is the Sun that is the Sun and this is Earth at that this is Earth at that position this is earth right over there if I were to draw if I were to kind of draw an arrow pointing straight out of the North Pole if I were to draw an arrow coming straight out of the North Pole it would look something like this 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 could go straight between the South Pole of the North Pole the angle between that and a pole that would actually be at a 90 degree angle or perpendicular to the plane of its orbit and so compared to if it was just straight up and down relative to the plane of the orbit so this right here is the angle of earth's tilt let me let me draw that a little bit bigger just so it becomes a little bit clearer so if this is the plane of the orbit we're looking sideways along the plane of the orbit and this is Earth right over here so this is Earth 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 what this is how earth would orbit this is how earth would rotate if it was Earth rotates 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 this is the South Pole and so it rotates and so it rotates it rotates like this and that angle relative to being vertical relative to being vertical with respect to the orbital plane this angle right here for Earth right now is 24 point is 20 sorry 23 point four degrees twenty-three point four degrees and if we're talking about relatively short periods of time like you know our lifespans that is constant but it is actually changing over long periods of time that is changing between and these are rough numbers is changing between twenty-two point one degrees twenty-two point one degrees and twenty-four point five degrees if the if my sources are correct but that gives a rough estimate of what is changing between but I want to make it clear this is not happening overnight the period before it to go from a roughly a 22 degree angle to a 24 and a half degree angle and back to a 22 degree angle is 41,000 years 41,000 41,000 years and this this long-term change in the tilt this might play into some of the long term climactic change maybe it might contribute 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 it as an angle of twenty three point four degrees now you might say okay I understand what the tilt is but how does that change the seasons in either the northern or the southern hemisphere and to do that I'm going to imagine the earth when it is tilt when it is most 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 in to relative to the rest of the universe if we assume that this tilt is at twenty three point four percent it's not changing throughout the year but depending on where it is in the orbit it's either going to be tilting away from the Sun as it is in this example right over here or it will be tilting towards the Sun or it would be tilting towards the Sun I'll do the tours the Sun in this magenta color or would be tilting towards the Sun so six months later when the earth is over here it's going that relative to the rest of to the rest of the universe it will be tilted in that same direction it will be tilted in that same direction up and up out of this page and to the right so out of this page 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 it tilt a little bit more towards the Sun if I were to draw it right over here if I were to draw it right over here it is now tilted it is now tilted towards the Sun it is now tilted towards the Sun and what I want to do is what I want to think about is how much day how much sunlight will different parts of the planet receive and I'll focus on the northern hemisphere but you can make a similar a similar argument for the southern hemisphere I don't think about how much sunlight they receive at when it's tilted away or tilted towards the Sun and so let's think about those two situations so first of all let's think about this situation here where we are tilted away from the Sun so let me zoom in a little bit so this is and this is the situation where we're tilted away from the Sun so if this is the vertical so let me draw it I could actually just use this diagram but let me make it so we're tilted away from the Sun like this we do this in a different color so if we have 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 are rotating and we are rotating around like that so int we're out of the page on the left hand side and then 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 that arrow is in the direction of the east so when we are at this point in Earth's orbit and actually let me copy and paste this and I'm gonna use the same exact diagram for this for the different seasons so let me copy and then let me paste this exact diagram I'll do it over here for two different points for two different points so when when we are 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 sunlight is coming from the left in this situation and so if you think about it what half of the earth is being or what part of the earth is being lit by sunlight or what part of the earth is in daylight the way I've drawn it right over here well the part that is facing the Sun so all of this all of this right over here is going to be in daylight when 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 so let me draw the equator which separates our northern and southern hemispheres so this is so this is the equator this is the equator and then let me go into the northern hemisphere and I want to 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 well if we go to the Arctic Circle so let me go 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 we'll just rotate around just like that it will just keep rotating around just like that and so my question is that point in the Arctic Circle is that rotates will it ever see sunlight well no it will never see sunlight because it the the North Pole is tilted away from the Sun so what I'm drawing what I'm shading here in purple what I'm shading here in purple that part of the earth when it'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 we are in this position in the orbit never I would I don't say never because once it becomes summer they will be able to see see it so they not no sunlight no day I guess you could say no daylight no daylight if you go to slightly more southern latitudes so let's say you go over here so maybe that's the latitude of something like I don't know New York or San Francisco or something like that let's think about what it would see as the Earth rotates every 24 hours so this would be daylight daylight daylight daylight then nighttime nighttime nighttime nighttime nighttime this is now going behind the globe nighttime nighttime nighttime nighttime nighttime daylight daylight daylight daylight so if you just compare this so let me do the daylight in orange so daylight is in orange and then nighttime and then nighttime I will do in this bluish purplish color so nighttime over here so if you go to really northern latitudes like the Arctic Circle they don't get any daylight when we're tilted away from the earth and if we go to slightly still northern out to latitudes but not as North is your Arctic Circle it does get daylight but it gets a lot less daylight it spends a lot less time in the than in the nighttime so notice if you if you compare the 20 if you if you say that this circumference represents the positions over 24 hours it spends much less time in the daylight than it does than it does in in the night time in the night time so because while the the northern hemisphere is tilted away from the earth it is the the latitudes in the northern hemisphere are getting less daylight so they're getting less daylight less daylight they're also getting less energy from the Sun and so that's what leads to winter or just being in generally colder and to see what happens in the summer let's just go the other side so now we're going to the other side of the our orbit around the Sun this is going to be six months later and notice the actual direction relative to the rest of the universe has not changed we're still pointed in that same direction we still have a twenty three point four degree tilt relative 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 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 and let me draw the equator again or my best attempt to draw the equator I'll draw the equator in that same color actually in that green color so this is the separates the northern and the 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 as 24 hours go around I'll keep rotating around here but notice the whole time I am inside of the Sun I'm getting no nighttime no there is no night in the Arctic Circle while we were tilted towards the Sun and if we still do that fairly north northern latitude but not as far as inner Arctic Circle maybe you know in San Francisco or New York or something like that if we go to that latitude notice how much time we spend in the Sun so maybe we just enter so this is this is right at sunrise and then as the day goes on as the day goes we're in sunlight sunlight sunlight sunlight sunlight sunlight sunlight sunlight then we hit sunset then we hit nighttime nighttime then we hit nighttime 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 versus sunlight you'll see it spends a lot more time in the daylight when the earth when the northern hemisphere is tilted towards the Sun so this is more more day more day less night last night so it is getting more energy from the Sun so when it's tilted towards the Sun it is getting more energy from the Sun so things will generally be warmer and so you are now you are now talking about summer you are now talking about summer in the Northern Hemisphere and the arguments for the southern hemisphere are identical you could even play it right over here when the northern hemisphere is tilted away from the Sun then the southern hemisphere is tilted towards the Sun and so for example the South Pole will have all daylight and no nighttime and southern latitudes will have more daylight than nighttime and so the South will have summer so this is summer in the south in 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 this is winter in the southern hemisphere and you might be saying hey Sal what about you're not you haven't talked a lot about spring and fall well let's think about it well if we're talking about the northern hemisphere this over here we decided 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 we're gonna get over here and then because of this tilt we aren't pointed away or towards the Sun we're kind of pointed we're pointed I guess sideways relative to the direction of the Sun but this doesn't favor one one one hemisphere over the other so when we're over here in and this will actually be this will actually be the spring now when 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 so as a spring this is the summer in the north hemisphere and now this will be this will be the fall in the northern hemisphere and once again we're tilted in this direction and so it's not the northern hemisphere has it tilted away or towards the or towards the Sun and so both hemispheres are going to get the same amount of radiation 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 night time because frankly that's maybe a little bit or at least in my brain a little bit easier to visualize but that by itself does not account for all of the difference between summer and winter another cause and actually this is probably the biggest 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's a certain amount of sunlight that is reaching the earth 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 in time at any point in time you see that much more of that is sitting the southern hemisphere than the northern hemisphere here all of these if you imagine it all of these rays right over here our sitting are hitting the southern hemisphere so a majority of the rays are hitting the southern hemisphere and much fewer hitting the northern hemisphere so actually a smaller amount of the radiation period at even a given period in time not even talking about not even talking about the amount of time you're facing the Sun but at any given moment in time more energy is hitting the southern hemisphere than the northern and this is the the opposite is true when the tilt is then towards the Sun now a disproportionate a disproportionate amount of the sun's energy a disproportionate amount of the sun's energy is hitting the northern hemisphere so if you draw a bunch of if you just think that this is all of the energy from the Sun most of it all of these rays up here are hitting the northern hemisphere and only these down here are hitting are hitting the southern hemisphere and on top of that what makes it even more extreme is that the actual angle that the and of course this is to some degree is due to the fact of where the the angle of the Sun relative to the horizon or where you are on earth but even more than that if you are on let's say say that this is the and 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 this is just what and we're looking this we're just looking at at the Sun here and over here you can see even when we are closest to the Sun the Sun is not directly overhead when we were close as the Sun the Sun still is pretty low on the horizon so maybe right over here when we're closest to the Sun in the winter the Sun might be right over here but if you look at that same latitude in the summer that same will attitude in the summer when it is closest to the Sun when it is closest to the Sun the Sun is more close to being directly overhead it still won't be directly overhead because we are still and we're where it's still at a relatively northern latitude but the Sun is going to be the Sun is going to be much higher the Sun is going to be much higher in the sky and these are all related to each other these are it's kind of connected with this idea that more energy is hitting is hitting the is hitting one hemisphere 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 you're it's actually going to be dissipated less by the atmosphere and let me just make it clear how this is so in the summer in the summer so let's say that that's the land and let's say that let me draw the atmosphere I'll draw the atmosphere in white so all of this area right over here this is the atmosphere and obviously there's not a hard boundary for the atmosphere but let's just say this is the densest part of the atmosphere in the summer in the summer when the Sun is higher in the sky in the summer when the Sun is higher in the sky the Rays from the Sun are dissipated by less atmosphere so they have to get through this this much atmosphere and they're bounced off and they heat some of that atmosphere before they but and they're absorbed before they get to the ground in the winter when the Sun is lower in the sky when the Sun is lower in the sky so maybe the Sun is out here let me draw it too a little bit so when the Sun is lower in the sky relative to this point you see that the rays of sunlight have to travel through a lot more a lot more atmosphere so they have a lot 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 reasons one is when you're tilted when you're tilted we'll say when you're tilted towards the Sun you are getting more absolute hours of daylight not only you're getting more absolute hours of daylight 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 opposed to the southern hemisphere and the stuff that's hitting the that's hitting the the places that have summer it has to go through less atmosphere so it gets dissipated less