Milankovitch Cycles Precession and Obliquity

How changes in Earth's rotation can effect Earth's seasons and climate

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, 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 the Sun 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 is if you think about the axis around it's rotating,

it's not straight up from the orbital plane.

It is at an angle.

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.

Maybe 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 right now it is at an angle of ...

it is at an angle of 23,4°

with the vertical, with being straight up and down.

And we've learned how this is what is the primary cause of our seasons,

and that when the Northern Hemisphere is pointed towards the Sun,

it's getting a disproportionate amount of the solar radiation,

whatever's going throught the atmosphere

has to go throught less atmosphere,

and that the things in the Northern Hemisphere are getting more daylight.

And when the Earth is on the other side of the Sun,

the Northern Hemisphere is pointed away from the Sun,

then the opoosite is going to happen

and the reverse is true for the Southern Hemisphere.

But in that video when we talked about how tilt can effect

the seasons,

I also 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 vary,

and even the boundaries for this varying are different

for the past million years

and they will be for the next million years,

but it varies roughly between 22,1° and 24,5°.

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 it exactly,

maybe pointing in this direction,

maybe 22,1° would look something like that,

in fact, I've exaggerated it,

and maybe 24,5° would go look something like that.

And so, it's not a huge difference but it is enough of 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 freeze over or not freeze over and all the rest,

or how much sunlight they get and 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.

41,000 years.

And right now, at a tilt of 23,4°

we're someplace right smack in between.

And we think the last minimum ... or sorry, the last maximum

was in 8700 BC, before the common era we could say, before Christ,

and that the next minimum, next time when our tilt will be minimized,

will be in the year 11,800.

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 fancier word for tilt is sometimes given is 'obliquity',

but this is really just a fancier word for tilt,

this changing of the obliquity or the 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 affects on climate

and maybe they do help cause certain Ice Ages

when they act together with each other over certain cycles.

And broadly, this entire class of cycles

are called Milankovitch cycles.

Milanković, he was a Serbian scientist

who was the guy who theorised that these changes of 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 changes in the tilt or the obliquity

are just one of the possible factors playing into Milankovitch cycles.

And what I wanna do in this video and the next fews

is to 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 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 in this direction

and obliquity tells you essentially how much it's wobbling.

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 appear

that's coming out of the pole,

that this was actually a physical arrow,

that that arrow itself would be rotating.

So, the best way to think about it is a wobbling top.

If you think that if at some point of time this thing would wobble,

so it would look like this.

Now the arrow is pointing that way.

If you wait a few more seconds,

now maybe the arrow is pointing out of the page.

And then you wait a few more seconds

and it's pointing in this direction, it's pointing into the page.

And so the whole time the obliquity isn't changing.

The obliquity you can kind of view as

how far is that wobble,

you can imagine how far from vertical is that wobble,

and the matter where we are in that rotation,

it hasn't changed,

and you can imagine it as a procesion,

as where we are in the wobble.

This is a little bit hard to visualise,

and hopefully as we think about it in different ways and I draw different diagrams

it'll become a little bit clearer.

But I wanna make it clear, just as it takes a long time

for the inclination to change from a minimum value to a maximum value and back.

It takes a huge amount of time

For Earth's procession to change in a significant way.

So, for this top to kind of,

if you imagine this arrow popping out,

for this arrow to actually trace an entire loop

it takes 26,000 years.

26,000 years to have an entire cycle of precession.

Now what I wanna do is think about

given that this precession is occuring,

I wanna think about how that would affect our seasons

or how it actually would 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 gonna think too much ...

I'm gonna assume that it's almost circular for the sake of this video.

In future videous I'll how the eccentricity or how eliptical the orbit is

can also affect the Milankovitch cycles,

or play into the Milankovitch cycles.

So let's just draw the orbit of Earth,

let me just draw the orbit of Earth around the Sun over here.

And so you can imagine this is at one pointed time, this is the Earth.

Let's say it is tilted towards the Sun right now,

so it is tilted towards the Sun.

So in the Northern Hemisphere,

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 have no precession, absolutely no precession,

when you go to this time over here,

you still have the same direction of tilt.

Let me do that in blue.

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.

You go to this tine, we're still tilting in the same direction relative to the universe

but we're now tilting away from the Sun,

and now this would be the winter in the Northern Hemisphere.

And we keep going around, if you had no precession,

when you get back to this point over here,

we'd be tilted in the exact same direction.

If your obliquity, if your tilt change 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 wanna 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,

but 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 little bit bigger.

So this is the Earth

and this is that arrow.

And this is hard to visualize,

at least it's hard for me to visualize.

Once you get it it's easy to visualize,

but the first time I tried to understand it,

it was hard to 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 at that same direction every year.

Now, with just precession alone what happens is

every year 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 in that circle.

So 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 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 in that point.

Now we would be most inclined to the Sun a little bit earlier in the year,

or a little bit earlier in the orbit.

So we would be most inclined to the Sun

maybe over here.

And it would take a many many actually 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 to the Sun.

So what the real effect of precession 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, that's slowly tracing out this circle

with, I guess you could say with its pole.

What it's doing is that it's making the tilt

towards the Sun or away from the Sun

a little bit earlier each year.

I know it's hard to visualize, but you can even take a top out

and have a basketball as the Sun,

and if you play with it, you'll see how that works.

And precession is another one of those factors

that affect, that play into I should say, the Milankovitch cycles.

What we'll see is when you combine precession,

or I shloud say changes in precession,

when you have combined that with changes in tilt

and you combined that with changes in actuall

how circular or how eliptical the actuall orbit is

and how that changes,

then you might have a respectable way of explainig

or some of explaining

why Earth does enter into these climatic cycles

over many tens of thousands of years.