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Current time:0:00Total duration:12:04

Video transcript

of in the video where we first introduced the Milankovitch cycles where we talk about the precession and how the the tilt of the earth the obliquity can also change I get this comment here on the YouTube channel from vixx oma and he or she says if I understood this correctly precession changes the time of the seasons over long periods of time and obliquity changes the strength of the season over long periods of time and so this is a good comment so the first comment isn't exactly true that's what I'm going to focus on in this video he says or she says precession changes the time of the seasons over long periods of time this is kind of true but I don't think in the sense that vixx OMA is referring to it the second part is roughly true o blick what changes the strength of the season over long periods of time if you are more tilted towards the Sun in the extreme then yes you will have a bigger disparity then when you are less tilted when you are tilted away from the Sun or I guess you'd say if you are more tilted to or away from the Sun the disparity between summer and winter will be greater than if you are less tilted so the second statement is true although although you always have to be careful with things as complicated as the climate because it can really depend from parts of the globe to parts on the globe depending on what are all of the other factors that play into it so you would want to run some type of simulation or anything or something like that but the second part is roughly true but I want to focus on the first part because I think it'll really give us a better understanding of what precession is and so this last statement vixx OMA makes is actually not true so he says I'm assuming that he means the Shi so in a several thousand in several thousand years not a in several thousand years if we still use the same calendar system summer in winter will happen in different months and they will be more mild or more harsh what we're going to see is that the second statement is not true because our calendar is actually based on when we are most tilted away or away towards our way from or towards the Sun so our calendar is actually based on is actually to some degree you could say takes precession into account and what actually does change is when we are closest what actually does change according to our calendar is when we are closest or furthest away from the Sun the perihelion or the aphelion and we're going to think about that in this video so let me draw the Sun here let me draw the Sun right over here and let me draw Earth's orbit around the Sun and I'm going to draw it with some eccentricity and just so you have know what I'm talking about when I say eccentricity a circle has no eccentricity and the ellipse this ellipse right here has more eccentricity than the circle which has no eccentricity and an even more eccentric ellipse would look like this so you can really think about eccentricity as a measure of how far you are away from being perfectly circular so Earth's orbit around the Sun is pretty close to circular but it has some eccentricity it is slightly elliptical and I'm going to exaggerate it a bit in this in this drawing right over here so let's say that this is the closest point that Earth gets to the Sun so that is the perihelion and let's say that this is the furthest point and obviously it's not this big of a difference it's actually only a 3% difference right now but we'll also learn that that's changing but it never becomes this dramatic but this will help us visualize it so Earth's orbit might look something like this or its orbit let me make it do a better job than that Earth's orbit might look something something like this obviously I'm exaggerating though the eccentricity here so let's say that's Earth's orbit this is the point in orbit where we're closest so that's perihelion perihelion and this is when we refer this this is f fillion aphelion aphelion and we saw in the first video when we discuss this that right now this perihelion is occurring in January and this will change over time as we'll see in this video so January right now and aphelion right now occurs in July now the time where we are most tilted towards the Sun is not at the perihelion right now it actually occurs a few weeks before the perihelion so when we are most tilted to the Sun this is our winter solstice this is our wilt winter solstice and this is when we are actually in the case of the northern hemisphere this is when we are most tilted away from the Sun I should say so if we were to draw our tilt here if I were to come out of the North Pole it would look like that at and this right over here depending on the year and where you our and your time zone and everything this is usually December 21st or 22nd I'll just go with December 22nd for now and when and when we are the northern hemisphere is most tilted towards the Sun occurs on June 20th or 21st so roughly six months later or really exactly six months later it's just that all the months have different time and have different have different numbers of days and and you have leap year sometimes with February sometimes having 28 or 29 days but if you go half a year away then you are at the you are at in the case of the northern hemisphere the summer solstice and this is when we are most tilted towards the Sun this is when we are most tilted towards the Sun and once again it occurs right now a few weeks before the aphelion before we are furthest away from the Sun now I want to zoom in right over here on on on December 22nd so right over here let me zoom in so let's say that this is the earth this is the earth I'm zooming in on this circle right over here so let me box it to show this is the one I am focused on right over here and let me draw let me draw let me draw the axis of rotation and we know that that angle versus the vertical that you could call the tilt or the obliquity and we know this is twenty three point four degrees relative to the vertical relative to perpendicular compared to the act the plane of our orbit I guess so if we if we were if we were completely if if our orbital axis was straight up and down it would look something like this it's not it tilts and this angle right over here is twenty three point four degrees and when I say straight up and down I'm saying relative to the plane of Earth's orbit around the Sun so this right here is the obliquity and as as vixx Allah mentioned this does change it kind of goes between 22 degrees and 24 and a half degrees over over very long periods of time but it does I think it's 41,000 years if I remember that correctly so it will affect on some level the severity so this tilt is kind of going between that we're actually it's reducing right now and it'll get to a minimum in in a few thousand years so it'll get to some minimum and then it might pretend then eventually it'll get back to some maximum tilt so it kind of goes back and forth but between those two over over over the course of several tens of thousands of years but anyway this is as it is zoomed in right now and as we mentioned precession you can kind of view it as if you you could kind of view it as if this if this arrow right here actually existed it would it would trace out it would trace out a circle and it's tracing out the circle over a huge period of time over 26,000 years so let me make everything clear here right now I'm just going to assume earth is rotating in that the orbital Direction is in that direction I'm going to assume make that a little bit more curved earth this is the rotation of the earth it is in this direction right over here and what we learned about procession and actually to be particular its axial precession there's multiple types of precession we'll talk about if someone says just precession they're usually referring to axial precession it's this idea that over 26,000 years the tip of this arrow or you can even imagine the poles themselves will kind of trace out a circle if you look at the same point in our orbit at any period in times the circle is tracing out is going to be going in this direction right over here in this direction right over here so if we wait 1,800 years and I want to make sure I get this right because it's important to see what happens to our calendar if we wait 1800 years we might this this this arrow instead of it'll still have a tilt of point three point four degrees but instead of pointing in this direction it might be pointing in this direction or in fact it it is likely it will be pointing in this direction I'm obviously not drawing it that exact and then the bottom of the hour will come out over here so if you think about that if you wait 1800 years and once again the tilt hasn't changed or has changed a little bit but what's what the precession has done tracing out the circle has changed the direction of this arrow change the direction of our axis of rotation and if you wait 1800 years when will when will be pointing or when will the Northern Hemisphere be pointed most away from the Sun well now it won't be pointed most away from the Sun at this point in space relative to the Sun anymore because now it's axis of rotation looks something like this looks something like this so now if we wait organized to say in 1800 years it'll be most pointed away or the northern hemisphere will be most pointed away from the Sun about a month earlier so about a month earlier it'll be most pointed away from the Sun about a month earlier so this is when it will be most pointed away from the Sun but if we - today's time we would say no that's still not the most pointed away but since we have this precession since the direction of the of the tilt I guess we could say or the the direction of our axis our rotational axis is changing we are now we we are now at a different point in orbit where we're most pointed away from the Sun and so this is 1800 years later 1800 years approximately 1800 years later so now based on this and I think this is what Vic's almost might might have been hinting at you say look okay it's earlier on our orbit wouldn't this now be like wouldn't this now be like November and the answer is no it will still be December 22nd this will still be this will still be December 21st or 22nd depending on the year still be December 22nd still will be the same date and that's because our calendar is based on when we are most tilted away or when we are most tilted towards the Sun so by definition this is when we're most tilted away so this will be the winter solstice so what happens is every year so the way I drew it right over here and actually this perihelion actually changes over time as well there's a precession of the perihelion as well but I'm not going to go into that right now so if you fast-forward 1800 years all that's going to happen is that what we consider by our calendar to be December 22nd in an absolute point in in our orbit will be earlier in our orbit but we're still going to call it December 22nd and so the perihelion is going to be further away from that December 22nd it's actually going to be a month further away so the perihelion eighteen hundred years from now won't be in January it will be it will be in February so the real takeaway here is that our calendar is based not on the exact point in space relative to the Sun our calendar is based on the the maximum tilt towards or away from the Sun and that as we see that that is slightly changing in terms of where it occurs on the absolute point in space I think it's changing by roughly 20 minutes a year so every year the perihelion is getting 20 minutes later if we wanted to use the perihelion if we wanted to use the exact point in space if we wanted to use the exact point in space as our calendar our year would actually be about that I don't know what it is roughly 20 or 25 minutes longer but it actually makes a much more sense to think about it from the tilt because that's what dictates the seasons and that's actually what's most observable from Earth where earth where the Sun is in the sky