Ep. 779: Milankovitch Cycles

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Astronomycast Episode 779

Milankovic Cycles

Welcome to Astronomycast, our weekly facts base

during through the cosmos where we help you understand

what we know, but how we know what we know.

I'm Fraser Kane, I'm the publisher of Universe today.

With me is Dr. Pamela Gay, a senior scientist

for the planetary science institute

and the director of CosmoQuest.

Hey Pamela, how are you doing?

I am doing well.

Next week we will not be recording live

because I'm actually going on a vacation.

Right on.

So I have been going through old media

that I recall loving.

Yes.

I watched recently and so I think I might have

mentioned people that I rewatch

the original Lord of the Rings trilogy,

the ones with the extended editions,

in the box set with the four discs per...

Yep.

Man, they're so great.

Obviously the movies were great, but just

that we've entered this world where nobody wants

to see behind the scenes, to hear the commentary,

to see the additional deleted scenes.

I don't understand that.

Yeah, like as a media creator,

I want to see how the sausage gets made.

Yeah, exactly.

Being able to see all of these little feature

ettes that they publish in addition to that

is so wonderful.

And I'm guessing that Netflix tried this

and just found that people were watching them

and they're like, pff, who cares?

We don't even need any of this stuff anymore.

Just get the movie and get in, get out.

So Disney Plus does do them for their Star Wars shows.

So each of the Star Wars shows

is behind the scenes special.

Yes.

And those are actually really cool.

They make me want to go try there.

They have this giant room that they record in

that allows them to see that.

Yeah, it's amazing.

So keep doing that.

And then I watched the Matrix series

and they held up, especially the first one.

Like the first one, there's a few bits

in the beginning where I was like,

oh, cringe.

But then the rest of the movie I thought was absolutely terrific.

Two movies I know a lot of people,

although I really dislike the ending of the third movie.

And then I rewatched Moon,

which is with Sam Rockwell,

and where he's an astronaut on the moon

running a mining operation.

And it's such a great movie.

Oh, it's so good.

And that was the long time since I watched that movie.

And so I'd forgotten most of the details.

And so I'm really enjoying

realizing that there are movies that I have enjoyed

in the past.

And now my,

I don't remember the details from scene to scene.

And so it's kind of like I'm rewatching it fresh.

And yet I know that this is going to be

a guaranteed great movie.

So if you haven't already,

go back, find movies

that it's been so long

that you probably don't remember the details.

But you know you're going to love them.

Television shows.

Blade Runner is so good.

Yeah, I watched, I watched Blade Runner

with my wife probably about six years ago

with that sort of same idea.

And then we watched the, the 2049.

Yeah.

But now I don't even remember how 2049 went.

So it's time to watch it again.

So there you go.

That's my recommendation.

Go back, find some old media that you're kind of nostalgic about.

But you're holding off because I've seen that movie.

Have you, do you remember all those neurons?

These are the modern political context.

There you go.

It's just a few short lives.

And from our perspective,

the seasons are something that come and go

with perfect regularity.

But astronomers know the terrible truth.

And that there are cycles that slowly shift

over tens of thousands of years.

Shifting the cycles and the Earth's climate.

Today we'll talk about the Milan-Kavitch cycles.

But first it's time for Brick.

And we're back.

So I think

we're going to need another disclaimer.

And that is that all of the cycles that we're going to talk about

and their influence on the Earth's climate

happen very slowly.

Yeah.

And they have absolutely no connection

to the climate change that

scientists are currently measuring.

It's also true.

Right.

So they are, you know,

that we are sort of forcing

climate change because of the greenhouse gases

that are being emitted by our martin civilization

in a way that is very easily calculated.

You can literally do this on the back of an envelope,

calculate, you know, take the volume of the mass,

the volume of the Earth's atmosphere,

measure the amount of carbon dioxide that's in it.

You can calculate the average global temperature

that will be expected at various concentrations

and there will be sort of specific differences

depending on whether it's the argument.

The complexity comes in all the simulations so on.

But you can just do this math.

So like, I know like some of you

are going to say, I'm never going to listen to astronomy cast again.

That's fine. You know, like you're going to

do what you got to do.

I know that some of you are going to be sort of

angry that we're not sort of, or the

you're hoping that we are going to somehow

turn this into an explanation for climate change.

And we are not.

We are just that these unfold on cycles that are much longer.

So with that,

and I don't want to like dance around.

No.

The reality of the astronomy and the geology

and the both of that.

Both things can be true at once.

Yes, right.

There can be ongoing orbital,

kinematic cycles.

Yes.

And there can be anthropogenic climate change.

Yes.

Both can be true.

Yes.

And then the Melanchovich cycles be

the explanation for climate change.

No. People checked.

Yeah. So there you go.

And like we don't need to go into that.

Like there's a whole other world

where there's all kinds of papers where people are checking

and it has no effect.

So let's get on then.

To the Melanchovich cycles.

With this sort of like, hey, it's 1971.

And we've never sort of experienced

the modern political diatribe

the results of climate science work.

And people just want to learn about the kinematics

and orbital interesting things that happen with planets

going around star.

So let's talk about the cycles.

So first you have to go all the way back to the 1920s,

which is really cool to me.

But do you have to go back to like the before the BCE?

Like you have to go back to the Greeks?

Yeah.

I mean, you could.

I want to.

I've actually known about

procession for a long time.

Because it folks like told me that we're looking back

through historic calendars of stellar positions.

We're able to track the changing North Pole.

Right.

It was more complicated than yeah.

I just oversimplified a whole lot of stuff there.

I'm going to rabbit hole for one second.

Yes.

Which is that the ancient Greeks

suspected that the earth orbits around the sun.

Yeah.

And that the proof that they that they would use

to confirm that was that they were looking

for a stellar parallax.

Yeah.

But.

And we know now that that parallax is there.

But you know, essentially the shifting of the view

of the stars based on the position of the earth,

whether it's on one side of the sun or the other.

Right.

That's how you tell that the earth is going around the sun.

You know, it says background is sort of shifting back and forth

every year.

Yeah.

And they knew that that's what you would expect to see.

And they tried to find it.

But all they had were these little siding tubes that they used.

That they would align with a star at the right time

and the right place.

And they couldn't measure to the level of precision

that was required to be able to confirm the stellar parallax.

And so they went down.

They never know the earth is the center of the universe.

Yeah.

Amazing.

The great thing was amazing.

Yeah.

What they could what they were able to do.

Anyway.

They had their suspicions.

They were already starting to detect the motion of the stars

in the sky year after year.

They just underestimated the size of the universe.

And when you underestimate the size of the universe,

you expect the parallaxes to be much,

much bigger than it actually is.

But they did see other motions in the sky over time.

And that's just super cool.

Yeah.

There's neat stories related to the salinity of the oceans

that you can go find.

There's all sorts of cool stuff.

Now it was in the 1920s that folks tried

to like mathematically take this on as geophysics began

to be an actual field.

So the 1920s is when astrophysics was born,

when geophysics was born,

when we started taking all of these things that we observed

and having enough understanding of math to be able to say,

well, this is why we see these things we see.

And they had started to realize at this point

that there had been geological cycles with racial periods.

Europe has lots and lots of evidence for glaciers.

North America has evidence for glaciers.

And so a Serbian scientist,

Milutin Molankovik,

in the 1920s, worked through James Crowell's earlier work,

trying to do all the maths by hand to figure out,

all right, what are the different motions we need to consider?

And so you have things like our Earth's orbit

is mostly circular, but not perfectly circular,

so at the beginning of January is when the Earth is closest to the Sun.

It's a fairly small effect, but it adds up.

And over the course of millennia,

Jupiter and Saturn's influence causes the orbit

to get slightly more elliptical and slightly rounder,

and slightly more elliptical and slightly rounder.

We're actually heading towards a rounder phase right now,

but the degree of change this causes

is several days per season of asymmetry between the seasons.

So here in the northern hemisphere,

because you move faster in your orbit when you're closer to the Sun,

you have a slightly shorter winter,

we are getting slightly more sunlight,

so the summer to winter dichotomy in the north

is less than is in the south,

and we literally get less winter in the north,

so that is excellent.

Some day the orbit will be much closer to round,

and it will essentially be equal seasons for everybody,

but we're not there right now.

We're headed that direction.

And how long does that cycle take?

So this is one of the cycles that isn't as periodic

as others, because you're dealing with Jupiter's orbit,

and you're dealing with Saturn's orbit.

So when you're looking up the extremes,

the extremes occur on millions of years.

So like the highest-eastern trustee ever

was 250 million years ago.

And that's like the Earth's orbit was the most elliptical

that it could be.

Yeah, yeah.

And so in general,

there's a 400,000-year coupling

and it's paired up with the 100,000-year cycles

that we see from other factors.

Right.

So about every 100,000 years,

you get a sort of a full cycle through the Earth

moving from what is a more circular orbit

to a more elliptical orbit.

And as you mentioned,

we experienced these seasons.

The North experiences shorter winters longer summers,

the South experiences longer winter shorter summers.

And partly,

that's through the eccentricity.

Essentially, Earth is at its closest point

to the Sun when the Northern Hemisphere

is getting the heart of its winter.

Yeah, yeah.

It's wild.

Yeah, it's incredible.

And so I know winter may feel like it sucks

for the Northern Hemisphere,

but it could be worse

consider the people in the Southern Hemisphere.

And one of the things we see is that the Arctic

can have extremely cold temperatures,

but Antarctica has ludicrously cold temperatures.

So yeah, so weirdly,

the Southern Hemisphere experiences these more severe winters

than we experience in the North.

And there's other weird stuff on top of this,

like the fact that there's so much land in the North.

Yes.

That has all sorts of weird effects

because oceans are thermal sinks

and land is capable of varying much faster.

So we know there's other complicating factors on top of this

and that's what makes it awesome.

And so this shift is driven by the interactions

of the gravity from Jupiter and Saturn

on the Earth's orbit,

tugging it bit by bit by bit,

orbit after orbit,

orbit slowly circularizing its orbit

and then slowly making its orbit more eccentric again.

All right, we're going to talk about this some more,

but it's time for another break.

And we're back.

So that is the first cycle.

Yes.

Let's move on to book two

of the lack of itch cycles.

So that is describing

the overall shape of our orbit.

Now, once you have the shape of the orbit

to contend with,

you have the Earth's position within that orbit,

by which I mean the way we're tilted.

So we're quite lucky right now

that for us here in the Northern Hemisphere,

I mean we have winter solstice

so close to a perihelian.

It didn't have to be that way.

And the tilt that we have right now

isn't hugely problematic.

It's 23.4 degrees.

And that tilt will vary over time

from 22.1 to 24.5.

So we have this tilt that's varying.

And we have the whole thing

is rotating.

So the amount that we're tilted

can get more extreme,

which makes the seasons more extreme.

Right.

And where we're pointed relative

to the stars will change.

And the tilt is changing

on a 41,000 year cycle.

Okay.

All right.

So imagine this imaginary line

that's passing through the poles of the Earth.

And the Earth is spinning around this

imaginary line.

Yeah.

And if you sort of look at the angle

of what that line is,

it's sort of slowly drifting down

and slowly drifting up,

slowly drifting back and forth,

and over this 41,000 years.

And so during that 41,000 year period,

you'll have the point of the maximum thing

and then it will go through the minimum

and then return to the maximum.

And that's sort of your cycle,

your 41,000 years.

And so we're at 20,

he's at 23.4.

Yeah.

So we're kind of smack in the middle

of a potential axial tilt.

It's perfectly reasonable.

A more extreme axial tilt

or a less extreme axial tilt.

And I guess if we had zero axial tilt,

then there would be no seasons.

You would just,

everything would be the same,

every day forever.

And, and then on top of this,

that entire tilt is rotating.

It's, it's procession like the procession of a top.

And that is happening on a 25.7,000 year cycle.

So every 25,700 years.

Yeah.

That is rotating.

And so that change,

changes when the seasons are occurring throughout the year.

Right.

And, you know, like I think,

we all learned, hopefully,

in elementary school or whatever,

like how the seasons work.

But, you know, I'm sure a pop quiz.

When you ask somebody,

how the seasons work,

but essentially,

a lot of people have no idea.

It's kind of amazing.

Yeah.

But it just is like,

again, imagine the earth.

It's this ball that's spinning.

It's tilted at an angle of 23.5 degrees.

During the summertime,

the northern hemisphere is tilted towards the sun.

Therefore,

it's experiencing more sunlight.

And the southern hemisphere is experiencing less sunlight.

And then when the situation is reversed,

then it's the southern hemisphere

that's experiencing more sunlight.

And it's the northern hemisphere that's receiving less.

But that in addition to that kind of angle changing,

you also get this wobbling of the top.

And as you said,

the seasons reverse

over the course of about 26,000 years.

And I,

I don't know what you,

you know,

I'll bet you we did the same thing.

Think back to young Pamela

in high school or elementary school,

learning this baffling fact.

And doing the math to figure out

how quickly the days change.

Did you do that?

No.

I did.

Oh, man.

I was like, wait a minute.

That means that like since the Roman time,

the seasons have shifted by some number of days.

And I, you know, calculated.

I forget what.

Like, you know,

a handful of days.

Like a couple of weeks have shifted.

That, yeah.

That summer has arrived.

I don't know,

earlier later.

I forget which way it goes.

By, by a measurable number of days.

Yeah.

Then what they used to experience.

These are things that have to get taken into account in archaeology.

Yeah.

And so when you're trying to consider like historical reports of the seasons

that things occurred from enough thousands of years ago,

slight changes that when you're dating things based on what flowers are in bloom,

that's the kind of stuff that kind of matters.

Totally.

And it's just wild to think about all the different changes that are going on.

And it all comes down to just a little bit of torque here,

a little bit of torque there.

It's all about uneven forces over time.

And so what's driving the, the wobble?

So the, the, it all comes down to the earth isn't a perfect sphere.

And the sun and the moon's gravitational force.

Just do a little bit more yanking where there's a little bit more stuff to yank on.

And that adds up to create this, this torque that generates the,

the procession that we see.

Okay.

Well, we're going to continue on to chapter three of the Malinkovich cycle.

But first it's time for another break.

And we're back.

All right.

Let's move on to book three.

That was my favorite book in the trilogy of Malinkovich cycle.

No, I have no opinion.

So on top of this, you have the, the earth's ellipticity means that,

that there's a, a axis of the orbit that is longer and an axis of the orbit that is shorter.

And where those axes are are rotating over time.

And so you have how much the earth's orbit is round changes over time.

You have the tilt changes over time.

You have where the tilt is pointed, changing over time.

And you have where the orbit is pointed, changing over time.

And, and yeah, it all adds up to change.

You have to keep track of both.

Where is the, the nearest point to the sun?

Where's the furthest point to the sun relative to the stars?

And also where is the North and South pole pointed relative to the stars?

So that you can figure out how do the solstices and perihelian and apahelian are?

Right.

Or fail to line up.

And so what, what impact did this have historically on the climate of the earth?

Well, so it, it was expected that there'd be a order of tens of thousands of year cycle in the glacial period.

For reasons we have not figured out yet, there is a hundred thousand cycle in the glacial period.

And, and again, we're still trying to figure out what are the additional effects that were missed the first time around.

Like one of the effects that was missed the first time around is the tilt of the orbit relative to the moment of inertia of the solar system has changed over time.

Right. So, so sorry, yeah.

So like the earth is tilted off of the sun's orbit.

Yeah, the Earth's orbit is tilted off the Jupiter's defined plane that is kind of the primary plane of the solar system.

Yeah.

And so if you imagine you sort of hold your hand out and you kind of imagine like here's the orbit of the sun.

Here's the, here's the, the equator of the sun and then the Earth is like slightly tilted off that.

And we call that the plane of the ecliptic is where the Earth's orbit is.

That that, that tilt is kind of going up and down as well.

And that's, that's fairly new.

Like this would be a fourth chapter that Milankovitch wrote after.

But he didn't write it.

But he didn't write it.

But he did, you know, he wanted to make more money because the trilogy had done so well.

He would have written part four.

The orbital inclination.

Right.

But you know, he never got around to that, to that chapter.

So, um, but, but like isn't it really about that sometimes these forces, these, these positions average each other out that they do that you've got.

We're a little bit close, you know, we're, we're on average closer to the sun.

Things are kind of warmer.

But then we're on average like a little more tilted away that Northern hemisphere is more extreme.

But then, but then it's pushing in another way.

Yeah.

And things get bad when they all line up in the worst possible ways.

Right.

And so you get the coldest, the most extreme, all of these things start to compound on each other.

And then you can find those in the geologic record of like, oh, yeah, we had a really bad glacier.

Well, no, no kidding.

Yeah.

We had, you know, we, we, that all those variations are happening cycle after cycle after cycle.

And each one is having this independent experience, you know, cause to the earth.

And sometimes they all balance out.

And there's no change.

And other times each one is hits the extreme.

And then you get a severe glaciation or a very long warm period.

Right.

And the other issue that we have to deal with is there's probably stuff that we just haven't found in the geologic record.

Because for the most part, each new glacial period likes to just scrape away the evidence of the past glaciation period.

We periodically get things like asteroid impacts that kill the dinosaurs.

We get massive changes to the life forms on the planet that lead to massive anoxic events and stuff like that.

And so each of the things that is tied to massive die offs does its own part to the environment of our world.

So yes, human beings are changing our planet.

We are not the first life form to do this.

We are just the most recent to do this.

And, and so the geologic record is super complicated to couple with, with what caused what?

Because sometimes it's the volcanoes got angry and Siberia, all of it erupted because it could.

The Siberian traps.

Yeah, the Siberian traps is the most terrifying thing in the geologic record as near as I can tell.

Just saying, if you want to stay awake at night, go read about that.

Look at the deck in traps.

Yeah, that's another one that's terrifying.

So trying to understand, is it what have we missed?

What secondary facts are we missing?

What additional things do we need to take into consideration?

We're still learning all of this and that's what's so amazing.

Yeah.

And why we keep doing science is because there is so much stuff left to learn and hopefully to find in some rock out crop we haven't explored yet.

And this is the thing that folks who work in fossils are constantly discovering is there's going to be a new cave.

There's going to be a new rock out crop.

There's going to be a new something erodes and uncovers something amazing and we'll find answers over time.

So one interesting cycle that is sort of disconnected is that the solar system is moving around the galaxy around the Milky Way.

Yeah.

And that it has like the like the earth is going sort of up and down in its orbit compared to the sun.

The solar system bobs up and down in the Milky Way as it orbits around and that it's thought that maybe that the solar system is protected by the mutual magnetic fields in the interstellar medium.

And so there are times when maybe the as the solar system rises up above the galactic plane and it can experience more cosmic radiation.

And maybe that could have led to times of greater die offs on the planet.

So you know, although there's no sun that we are orbiting in the Milky Way that is giving us illumination that we need that defines the global temperature.

There can be particles that are colliding with the planet whether depending on our position above and below the galactic plane, which is a totally different cycle.

You know, I'm not necessarily super confirmed to be a thing, but it's kind of interesting to think about these even larger cycles.

It's all cycles within cycles.

Right. And that one is thought to also be tied to the sudden a whole bunch more comets get sent on their way into the interstellar system.

They also get sent out of the solar system they get sent in both directions.

Yeah. Yeah. Or like I said, or less protection from cosmic rays by some magnetic fields of all of the stars.

So yeah, it's a very interesting sort of idea to think about.

Cool. Well, there you go. The morning of its cycles. And now you know.

And now you know, thanks, Pamela. Thank you, Fraser.

And thank you so much to everyone on Patreon that supports us and allows us to keep putting these episodes together.

This week, I'd like to thank the following $10 a month and up patrons, Abraham Catrell, Alex Cohen, Andrew Allen, Andy Moore, Arno DeGroote, Bore Andro Levesval, Benjamin Carrier, Bill Smith.

Boogey Net, Brian Breed, Brian Kilby, Buzz Parsec, Claudia Maestriani, Cooper, Daniel Shecter, David Gates, Diane Philippon, Don Mundis, Ed, Eric Lee, Father Prax, Fredric Salvo, G. Calibre Sexton, Gerhard Schweitzer, Gold, Greg Viled, Hannah Takary, Jacob Hool, Jarvis Earl, Jeanette Wink, Jim McGeean.

Joanne Mulvey, John Muthus. Thank you so very much.

Thanks, Pamela. We'll see you next week.

Thank you, Fraser. Move on. I guess we won't.

No, next week we won't. Next week I'm going on vacation with friends.

Yeah, we'll see you in two weeks. Okay, bye-bye.

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