# Precession causing perihelion to happen later | Cosmology & Astronomy | Khan Academy

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In the video where we first introduced the Milankovitch cycles where we talk about the precession and how the tilt of the Earth, the obliquity, can also change, I get this comment here on the YouTube channel from Vicksoma 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 strengths of the season over long periods of time. And so this is a good comment. So the first comment isn't exactly true, and 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 Vicksoma is referring to it. The second part is roughly true. Obliquity changes the strengths 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 than 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 you always have to be careful with things as complicated as the climate because it can really depend from parts of the global 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 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 Vicksoma makes is actually not true.

So he says– I'm assuming it's a he, maybe it's a she– so in a several thousand years, in several thousand years– not a– in several thousand years, if we still use the same calendar system, summer and winter will happen in different months and they will be more mild or more harsh. And 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 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 according to our calendar is when we are closest or furthest away from the sun– the perihelion or 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 know what I'm talking about when I say eccentricity, a circle has no eccentricity. An ellipse, this ellipse right here has more eccentricity than this 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 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 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. Earth's orbit– let me make it, I can do a better job than that– Earth's orbit might look something like this.

Obviously, I'm exaggerating, though, the eccentricity here. So let's say that's Earth's orbit. This is the point where we are closest, so that's perihelion. And this is when we are furthest. This is aphelion. And we saw in the first video when we discussed 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. 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. And this right over here, depending on the year and where you are in your time zone and everything, this is usually December 21st or 22nd.

I'll just go with December 22nd for now. And when 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 numbers of days 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– in the case of the Northern Hemisphere– the summer solstice, and 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 a right over here on December 22nd. So right over here, let me zoom in. So let's say that 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 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 23.

4 degrees relative to the vertical– relative to perpendicular compared to the plane of our orbit, I guess. So 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 23.4 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 Vicksoma mentioned, this does change, it goes between 22 degrees and 24 and 1/2 degrees over very long periods of time, but it does– oh, 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 going between that where actually it's reducing right now, and it will get to a minimum in a few thousand years. So it will get to some minimum and then eventually it will get back to some maximum tilt so it goes back and forth between those two 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 view it as if this arrow right here actually existed, it would trace out a circle and is tracing out of this 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 and that the orbital direction is in that direction. I'm going to assume– let me make that a little bit more curved– this is the rotation of the Earth, it is in this direction right over here. And what we learned about precession– and actually, to be particular, it's axial precession. There's multiple types of precession we will 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 could even imagine the poles themselves, will trace out a circle. If you look at the same point in our orbit at any period in time, the circle that is tracing out is going to be going 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 1,800 years this arrow, it will still have a tilt of 23.

4 degrees, but instead of pointing in this direction, it might be pointing in this direction. Or in fact, it will be pointing in this direction. I'm obviously not drawing it that exact. And then the bottom of the arrow will come out over here. So if you think about that, if you wait 1,800 years, and once again, the tilt hasn't changed or it's changed a little bit, but what the precession has done, tracing out this circle has changed the direction of this arrow, it changed the direction of our axis of rotation. And if you wait 1,800 years 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 its axis of rotation looks something like this. So now if we wait– or I should say in 1,800 years, it will 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. To 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 tilt I guess we could say, or the direction of our rotational axis is changing, we are now at a different point in our orbit where we are most pointed away from the sun. So this is 1,800 years later, approximately. So now based on this, and I think this is what Vicksoma might have been hinting at, you say, look, OK, it's earlier in our orbit. Wouldn't this now be November? And the answer is no. It will still be December 22nd. This will still be December 21st or 22nd Depending on the year. Still 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 are 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 1,800 years, all that's going to happen is that what we consider by our calendar to be December 22nd in an absolute point in our orbit we'll 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 1,800 years from now won't be in January, 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 maximum tilt towards or away from the sun, and that, as we see, that is slightly changing in terms of where it occurs in 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 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 little 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 the sun is in the sky.

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