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 this if this is the orbital plane right over here
so we're looking right directly sideways on this orbital plane
that I've drawn in orange.
And maybe at the point in Earth's orbit right
now, maybe the sun is to the left,
and so the rays from the sun are coming
in this general direction.
We've learned that the Earth as a certain tilt,
and when I mean that it means if you think
about the axis around which it's rotating it's not straight
up from the orbital plane, it is at an angle.
And 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.
And 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 23.4 degrees
with the vertical, with being straight up and down.
And we've learned how this is what
is the primary cause of our seasons
in that when the Northern Hemisphere is pointed
towards the sun it's getting a disproportionate amount
of the solar radiation.
Whatever's going through the atmosphere has
to go through less atmosphere and the things in the Northern
Hemisphere are getting more daylight.
And when the Earth is on the other side of the sun
and the Northern Hemisphere is pointed away
from the sun then the opposite is going to happen.
And the reverse is true for the Southern Hemisphere.
But in that video when we talk about how
tilt can affect the seasons, I also
kind of 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
than they will be for the next million years,
but it varies roughly between 22.1 degrees and 24.5 degrees.
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 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 look something like that.
And so it's not a huge difference,
but it is enough of a 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 degrees,
we're someplace right smack in between.
And we think the last maximum was at 8,700 BCE,
before the common era, or you could say before Christ.
And that the next minimum, when our tilt has been minimized,
the next time 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 fancy 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 effects on Earth's 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.
Milankovitch, he was a Serbian scientist,
who's the guy who theorized that these changes in 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 elbow obliquity,
are just one of the possible factors
playing into Milankovitch cycles.
And what I want to do in this video and in the next video
is 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.
And the idea behind precession, I
guess the best analogy I can think of,
is if you imagine a top, or maybe you
could imagine Earth as a top right over here.
The top is spinning, 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 is also if you imagine
that this was a pole up here 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 after some point of time
this thing would wobble so it would look like this.
So now the arrow is pointing that way.
And if you wait a few more seconds now
maybe the arrow is pointing a little bit out of the page.
And then you wait a few more seconds then
it's pointing in this direction, it's pointing into the page.
And so this whole time, the obliquity isn't changing.
The obliquity you can view it as how far is that wobble.
You could imagine how far from vertical is that wobble,
and no matter where we are in that rotation
it hasn't changed, and you could imagine the precession
as where we are in the wobble.
And I want to-- this is a little bit hard to visualize,
and hopefully, as we think about it in different ways
and I draw different diagrams it'll
become a little bit clearer-- but I want to 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 precession
to change in a significant way.
So for this top to-- if you imagined this arrow popping
out, for this arrow to actually trace out an entire loop,
it takes 26,000 years.
So 26,000 years to have an entire cycle of precession.
Now, what I want to do is think about
given that this precession is occurring,
I want to think about how that would affect our seasons,
or how it would actually 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 going to think too much--
I'm going to assume that it's almost
circular for the sake of this video.
In future videos, we'll talk about how
the eccentricity-- or how elliptical the orbit is--
can also affect the Malinkovitch cycles
or play into the Malinkovitch cycles.
But let's just draw the orbit of Earth around the sun over here.
And so you could imagine this is, at one point in time,
this is the Earth.
Let's say it is tilted towards the sun right now.
And so is in the Northern Hemisphere--
and 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 had no precession, absolutely
no precession, when you go to this time of year
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 time, we're still tilting in the same direction
relative to the universe, but we're not
tilting away from the sun.
And now this would be the winter in the Northern Hemisphere.
And we'd keep going around.
And if you had no precession, when you get back
to this point over here, we'd be tilting
in the exact same direction.
If your obliquity or if your tilt changed 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'm going to 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,
by the time you get to this point again
you're not pointing at exactly the same direction.
You are 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 or at least it's hard for me
to visualize.
Well, once you get it, it's easier to visualize,
but the first time I tried to understand it,
it was hard for 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 in that same direction every year.
Now, with just precession alone what happens is every year
this arrow is 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 of 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 at that point.
Now, we would have been most inclined
to the sun a little bit earlier in the year or a little bit
earlier in the orbit.
So we would have been most inclined to the sun
maybe over here.
And it would take many, many, many actual thousands
of years for the precession to change this much,
but then over here this is where when at this point in that year
when we would be pointed most towards 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 it's making it
tilt towards the sun or away from the sun
a little bit earlier each year.
I know it's hard to visualize, but you could 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
play into, I should say, the Malinkovitch cycles.
And what we'll see is when you combine precession,
when you combine-- or I should say change in precession, when
you combine that with changes in tilt,
and you combine that with changes in actual how circular
or how elliptical the actual orbit is and how that changes,
then you might have a respectable way of explaining,
or some of explaining, why Earth is entered
into these climactic cycles over many tens of thousands
of years.