What causes precession and other orbital changes

WEBVTT
Kind: captions
Language: en

00:00:00.840 --> 00:00:02.260
I've talked a lot
about precession

00:00:02.260 --> 00:00:04.890
and the changes in the
inclination of Earth's orbit,

00:00:04.890 --> 00:00:07.172
but I haven't told you
why they are happening.

00:00:07.172 --> 00:00:09.130
And I'm not going to go
into the physics of it.

00:00:09.130 --> 00:00:12.260
It's a little bit beyond this
discussion right over here.

00:00:12.260 --> 00:00:15.020
But they're really a
byproduct of the Earth

00:00:15.020 --> 00:00:18.560
and sun's interactions with
Earth and with the fact

00:00:18.560 --> 00:00:20.700
that Earth is not
a perfect sphere.

00:00:20.700 --> 00:00:24.490
So if I draw Earth-- so this
is my little drawing of Earth,

00:00:24.490 --> 00:00:27.130
and let me put the poles over
here, North Pole and South

00:00:27.130 --> 00:00:30.250
Pole, it actually turns
out that Earth is fatter

00:00:30.250 --> 00:00:31.700
than it is taller.

00:00:31.700 --> 00:00:34.490
So if you were to
measure Earth's diameter

00:00:34.490 --> 00:00:43.240
along the equator,
it is 43 kilometers,

00:00:43.240 --> 00:00:48.040
which is approximately 27
miles longer than if you

00:00:48.040 --> 00:00:50.440
were to measure its
diameter from pole to pole.

00:00:50.440 --> 00:00:53.530
So longer than the
pole to pole diameter.

00:00:53.530 --> 00:00:57.590
And the fact that Earth has this
equatorial bulge, that it's not

00:00:57.590 --> 00:00:59.280
a perfect sphere--
and once again, I'm

00:00:59.280 --> 00:01:00.780
not going to go
into the math here--

00:01:00.780 --> 00:01:02.740
it's the interactions
between that,

00:01:02.740 --> 00:01:07.040
I guess you could call it, that
one asymmetry of the Earth,

00:01:07.040 --> 00:01:08.950
it's that interaction
between that

00:01:08.950 --> 00:01:13.050
and the pull of gravity
between the Earth and the sun

00:01:13.050 --> 00:01:16.320
and the moon that
causes these long term

00:01:16.320 --> 00:01:23.260
cycles, this axial
precession and other less

00:01:23.260 --> 00:01:26.250
noticeable changes
in Earth's orbit.

00:01:26.250 --> 00:01:27.740
And as we'll see
in the next video,

00:01:27.740 --> 00:01:30.670
these aren't the only types
of changes in orbits we have.

00:01:30.670 --> 00:01:33.580
We also have changes
in the actual ellipse

00:01:33.580 --> 00:01:36.870
that Earth's orbit has
actually rotates over time.

00:01:36.870 --> 00:01:39.020
But that's due more
to interactions

00:01:39.020 --> 00:01:41.080
with Earth's orbit,
and the orbit

00:01:41.080 --> 00:01:42.622
of other planets in
our solar system.

00:01:42.622 --> 00:01:44.496
And once again, it's
one of those things that

00:01:44.496 --> 00:01:46.830
happened over thousands
and thousands of years.

00:01:46.830 --> 00:01:49.330
So all of these changes,
they're because Earth

00:01:49.330 --> 00:01:53.230
isn't completely symmetrical,
more fat than it is tall,

00:01:53.230 --> 00:01:55.680
those interactions
between the gravity

00:01:55.680 --> 00:01:57.410
of the sun and the moon.

00:01:57.410 --> 00:02:01.440
And Earth's orbit, as a whole,
changes because of interactions

00:02:01.440 --> 00:02:05.950
with other objects inside
of our solar system.


Precession causing perihelion to happen later

WEBVTT
Kind: captions
Language: en

00:00:01.120 --> 00:00:03.680
In the video where we first
introduced the Milankovitch

00:00:03.680 --> 00:00:05.920
cycles where we talk
about the precession

00:00:05.920 --> 00:00:09.630
and how the tilt of the Earth,
the obliquity, can also change,

00:00:09.630 --> 00:00:13.210
I get this comment here on the
YouTube channel from Vicksoma

00:00:13.210 --> 00:00:16.290
and he or she says, if I
understood this correctly,

00:00:16.290 --> 00:00:18.650
precession changes the
time of the seasons

00:00:18.650 --> 00:00:21.580
over long periods of time,
and obliquity changes

00:00:21.580 --> 00:00:24.550
the strengths of the season
over long periods of time.

00:00:24.550 --> 00:00:25.850
And so this is a good comment.

00:00:25.850 --> 00:00:27.524
So the first comment
isn't exactly true,

00:00:27.524 --> 00:00:29.690
and that's what I'm going
to focus on in this video.

00:00:29.690 --> 00:00:31.780
He says or she says
precession changes

00:00:31.780 --> 00:00:35.270
the time of the seasons
over long periods of time.

00:00:35.270 --> 00:00:37.980
This is kind of true, but
I don't think in the sense

00:00:37.980 --> 00:00:40.250
that Vicksoma is
referring to it.

00:00:40.250 --> 00:00:42.060
The second part is roughly true.

00:00:42.060 --> 00:00:44.160
Obliquity changes the
strengths of the season

00:00:44.160 --> 00:00:45.550
over long periods of time.

00:00:45.550 --> 00:00:49.000
If you are more tilted towards
the sun in the extreme,

00:00:49.000 --> 00:00:51.510
then, yes, you will
have a bigger disparity

00:00:51.510 --> 00:00:53.040
than when you are
less tilted, when

00:00:53.040 --> 00:00:54.414
you are tilted
away from the sun.

00:00:54.414 --> 00:00:55.980
Or I guess you'd
say, if you are more

00:00:55.980 --> 00:00:58.482
tilted to or away from
the sun, the disparity

00:00:58.482 --> 00:01:00.190
between summer and
winter will be greater

00:01:00.190 --> 00:01:01.420
than if you are less tilted.

00:01:01.420 --> 00:01:04.940
So the second statement is
true, although you always

00:01:04.940 --> 00:01:07.790
have to be careful with things
as complicated as the climate

00:01:07.790 --> 00:01:10.080
because it can really depend
from parts of the global

00:01:10.080 --> 00:01:11.980
to parts on the globe
depending on what

00:01:11.980 --> 00:01:14.050
are all of the other
factors that play into it.

00:01:14.050 --> 00:01:16.720
So you would want to run some
type of simulation or something

00:01:16.720 --> 00:01:18.729
like that, but the second
part is roughly true.

00:01:18.729 --> 00:01:20.270
But I want to focus
on the first part

00:01:20.270 --> 00:01:21.770
because I think
it'll really give us

00:01:21.770 --> 00:01:23.860
a better understanding
of what precession is.

00:01:23.860 --> 00:01:25.980
And so this last
statement Vicksoma makes

00:01:25.980 --> 00:01:26.870
is actually not true.

00:01:26.870 --> 00:01:28.828
So he says-- I'm assuming
it's a he, maybe it's

00:01:28.828 --> 00:01:31.750
a she-- so in a
several thousand years,

00:01:31.750 --> 00:01:34.940
in several thousand years-- not
a-- in several thousand years,

00:01:34.940 --> 00:01:37.481
if we still use the same
calendar system, summer

00:01:37.481 --> 00:01:39.230
and winter will happen
in different months

00:01:39.230 --> 00:01:41.607
and they will be more
mild or more harsh.

00:01:41.607 --> 00:01:43.940
And what we're going to see
is that the second statement

00:01:43.940 --> 00:01:46.010
is not true because our
calendar is actually

00:01:46.010 --> 00:01:52.950
based on when we are most tilted
away from or towards the sun.

00:01:52.950 --> 00:01:55.500
So our calendar
is actually based

00:01:55.500 --> 00:01:57.440
on-- is actually
to some degree you

00:01:57.440 --> 00:01:59.910
could say takes
precession into account,

00:01:59.910 --> 00:02:05.450
and what actually does change
according to our calendar is

00:02:05.450 --> 00:02:07.780
when we are closest or
furthest away from the sun--

00:02:07.780 --> 00:02:09.389
the perihelion or aphelion.

00:02:09.389 --> 00:02:12.190
And we're going to think
about that in this video.

00:02:12.190 --> 00:02:14.450
So let me draw the sun here.

00:02:14.450 --> 00:02:16.260
Let me draw the sun
right over here.

00:02:16.260 --> 00:02:18.340
And let me draw Earth's
orbit around the sun.

00:02:18.340 --> 00:02:20.340
And I'm going to draw it
with some eccentricity.

00:02:20.340 --> 00:02:22.340
And just so you know what
I'm talking about when

00:02:22.340 --> 00:02:25.560
I say eccentricity, a
circle has no eccentricity.

00:02:25.560 --> 00:02:28.300
An ellipse, this
ellipse right here

00:02:28.300 --> 00:02:29.930
has more eccentricity
than this circle,

00:02:29.930 --> 00:02:31.530
which has no eccentricity.

00:02:31.530 --> 00:02:35.490
And an even more eccentric
ellipse would look like this.

00:02:35.490 --> 00:02:37.250
So you can really think
about eccentricity

00:02:37.250 --> 00:02:40.330
as a measure of how
far you are away

00:02:40.330 --> 00:02:41.960
from being perfectly circular.

00:02:41.960 --> 00:02:44.920
So Earth's orbit around the sun
is pretty close to circular,

00:02:44.920 --> 00:02:46.410
but it has some eccentricity.

00:02:46.410 --> 00:02:48.200
It is slightly
elliptical, and I'm

00:02:48.200 --> 00:02:51.860
going to exaggerate it a bit in
this drawing right over here.

00:02:51.860 --> 00:02:55.140
So let's say that this is the
closest point that Earth gets

00:02:55.140 --> 00:02:57.900
to the sun so that
is the perihelion.

00:02:57.900 --> 00:02:59.620
And let's say this is
the furthest point.

00:02:59.620 --> 00:03:01.706
And obviously, it's not
this big of a difference.

00:03:01.706 --> 00:03:03.580
It's actually only a 3%
difference right now,

00:03:03.580 --> 00:03:05.910
but we'll also learn
that that's changing.

00:03:05.910 --> 00:03:07.730
But it never becomes
this dramatic,

00:03:07.730 --> 00:03:09.600
but this will help
us visualize it.

00:03:09.600 --> 00:03:12.100
So Earth's orbit might
look something like this.

00:03:12.100 --> 00:03:14.353
Earth's orbit--
let me make it, I

00:03:14.353 --> 00:03:16.840
can do a better job than
that-- Earth's orbit

00:03:16.840 --> 00:03:22.180
might look something like this.

00:03:22.180 --> 00:03:25.224
Obviously, I'm exaggerating,
though, the eccentricity here.

00:03:25.224 --> 00:03:26.640
So let's say that's
Earth's orbit.

00:03:26.640 --> 00:03:29.810
This is the point where we are
closest, so that's perihelion.

00:03:32.600 --> 00:03:34.010
And this is when
we are furthest.

00:03:34.010 --> 00:03:34.770
This is aphelion.

00:03:37.984 --> 00:03:40.150
And we saw in the first
video when we discussed this

00:03:40.150 --> 00:03:44.460
that right now this perihelion
is occurring in January,

00:03:44.460 --> 00:03:47.610
and this will change over time
as we'll see in this video.

00:03:47.610 --> 00:03:50.470
So January right now,
and aphelion right now

00:03:50.470 --> 00:03:53.150
occurs in July.

00:03:53.150 --> 00:03:57.590
Now, the time where we are
most tilted towards the sun

00:03:57.590 --> 00:03:59.770
is not at the
perihelion right now.

00:03:59.770 --> 00:04:02.260
It actually occurs a few
weeks before the perihelion.

00:04:02.260 --> 00:04:04.730
So when we are most
tilted to the sun,

00:04:04.730 --> 00:04:06.605
this is our winter solstice.

00:04:09.550 --> 00:04:12.090
And this is when
we are-- actually,

00:04:12.090 --> 00:04:14.110
in the case of the
Northern Hemisphere,

00:04:14.110 --> 00:04:16.370
this is when we are most
tilted away from the sun,

00:04:16.370 --> 00:04:17.000
I should say.

00:04:17.000 --> 00:04:19.886
So if we were to
draw our tilt here,

00:04:19.886 --> 00:04:21.510
if I were to come
out of the North Pole

00:04:21.510 --> 00:04:23.710
it would look like that.

00:04:23.710 --> 00:04:26.160
And this right over here,
depending on the year

00:04:26.160 --> 00:04:28.610
and where you are in your
time zone and everything,

00:04:28.610 --> 00:04:31.150
this is usually
December 21st or 22nd.

00:04:31.150 --> 00:04:33.610
I'll just go with
December 22nd for now.

00:04:33.610 --> 00:04:39.510
And when the Northern Hemisphere
is most tilted towards the sun

00:04:39.510 --> 00:04:45.620
occurs on June 20th or 21st,
so roughly six months later.

00:04:45.620 --> 00:04:47.160
Or really exactly
six months later.

00:04:47.160 --> 00:04:52.740
It's just that all the months
have different numbers of days

00:04:52.740 --> 00:04:55.450
and you have leap year sometimes
with February sometimes having

00:04:55.450 --> 00:04:58.390
28 or 29 days, but if
you go half a year away

00:04:58.390 --> 00:05:01.770
then you are at-- in the case
of the Northern Hemisphere--

00:05:01.770 --> 00:05:03.420
the summer solstice,
and this is when

00:05:03.420 --> 00:05:05.480
we are most tilted
towards the sun.

00:05:08.670 --> 00:05:11.780
And once again, it occurs
right now a few weeks

00:05:11.780 --> 00:05:17.260
before the aphelion, before we
are furthest away from the sun.

00:05:17.260 --> 00:05:23.730
Now, I want to zoom in a right
over here on December 22nd.

00:05:23.730 --> 00:05:26.300
So right over here,
let me zoom in.

00:05:26.300 --> 00:05:29.760
So let's say that
this is the Earth.

00:05:29.760 --> 00:05:32.240
I'm zooming in on this
circle right over here

00:05:32.240 --> 00:05:34.100
so let me box it to
show this is the one I

00:05:34.100 --> 00:05:36.170
am focused on right over here.

00:05:36.170 --> 00:05:42.780
And let me draw the
axis of rotation.

00:05:42.780 --> 00:05:46.020
And we know that that angle
versus the vertical that you

00:05:46.020 --> 00:05:47.840
could call the tilt
or the obliquity

00:05:47.840 --> 00:05:51.250
and we know this is
23.4 degrees relative

00:05:51.250 --> 00:05:54.520
to the vertical-- relative
to perpendicular compared

00:05:54.520 --> 00:05:57.250
to the plane of
our orbit, I guess.

00:05:57.250 --> 00:06:02.054
So if our orbital axis
was straight up and down,

00:06:02.054 --> 00:06:03.470
it would look
something like this.

00:06:03.470 --> 00:06:06.100
It's not, it tilts, and
this angle right over here

00:06:06.100 --> 00:06:08.280
is 23.4 degrees.

00:06:08.280 --> 00:06:09.780
And when I say
straight up and down,

00:06:09.780 --> 00:06:12.470
I'm saying relative to
the plane of Earth's orbit

00:06:12.470 --> 00:06:13.890
around the sun.

00:06:13.890 --> 00:06:15.390
So this right here
is the obliquity.

00:06:15.390 --> 00:06:18.290
And as Vicksoma mentioned,
this does change,

00:06:18.290 --> 00:06:22.100
it goes between 22 degrees
and 24 and 1/2 degrees

00:06:22.100 --> 00:06:26.640
over very long periods of
time, but it does-- oh, I

00:06:26.640 --> 00:06:29.530
think it's 41,000 years if
I remember that correctly--

00:06:29.530 --> 00:06:32.160
so it will affect on
some level the severity.

00:06:32.160 --> 00:06:34.410
So this tilt is going between
that where actually it's

00:06:34.410 --> 00:06:36.560
reducing right now,
and it will get

00:06:36.560 --> 00:06:40.250
to a minimum in a
few thousand years.

00:06:40.250 --> 00:06:42.864
So it will get to some
minimum and then eventually it

00:06:42.864 --> 00:06:44.280
will get back to
some maximum tilt

00:06:44.280 --> 00:06:47.320
so it goes back and
forth between those two

00:06:47.320 --> 00:06:52.760
over the course of several
tens of thousands of years.

00:06:52.760 --> 00:06:55.940
But anyway, this is as it
is zoomed in right now.

00:06:55.940 --> 00:06:58.770
And as we mentioned,
precession you can view it

00:06:58.770 --> 00:07:04.770
as if this arrow right
here actually existed,

00:07:04.770 --> 00:07:08.640
it would trace out a
circle and is tracing out

00:07:08.640 --> 00:07:10.860
of this circle over a
huge period of time,

00:07:10.860 --> 00:07:13.460
over 26,000 years.

00:07:13.460 --> 00:07:15.772
So let me make
everything clear here.

00:07:15.772 --> 00:07:17.980
Right now, I'm just going
to assume Earth is rotating

00:07:17.980 --> 00:07:21.607
and that the orbital direction
is in that direction.

00:07:21.607 --> 00:07:23.940
I'm going to assume-- let me
make that a little bit more

00:07:23.940 --> 00:07:27.920
curved-- this is the
rotation of the Earth,

00:07:27.920 --> 00:07:31.054
it is in this direction
right over here.

00:07:31.054 --> 00:07:33.220
And what we learned about
precession-- and actually,

00:07:33.220 --> 00:07:35.157
to be particular,
it's axial precession.

00:07:35.157 --> 00:07:37.490
There's multiple types of
precession we will talk about.

00:07:37.490 --> 00:07:39.250
If someone says just
precession they're

00:07:39.250 --> 00:07:41.480
usually referring
to axial precession.

00:07:41.480 --> 00:07:46.120
It's this idea that over 26,000
years, the tip of this arrow,

00:07:46.120 --> 00:07:48.490
or you could even imagine
the poles themselves,

00:07:48.490 --> 00:07:50.060
will trace out a circle.

00:07:50.060 --> 00:07:52.370
If you look at the
same point in our orbit

00:07:52.370 --> 00:07:55.080
at any period in time, the
circle that is tracing out

00:07:55.080 --> 00:07:59.570
is going to be going in this
direction right over here.

00:07:59.570 --> 00:08:01.040
So if we wait
1,800 years-- and I

00:08:01.040 --> 00:08:02.998
want to make sure I get
this right because it's

00:08:02.998 --> 00:08:05.060
important to see what
happens to our calendar--

00:08:05.060 --> 00:08:10.280
if we wait 1,800
years this arrow,

00:08:10.280 --> 00:08:12.410
it will still have a
tilt of 23.4 degrees,

00:08:12.410 --> 00:08:14.160
but instead of pointing
in this direction,

00:08:14.160 --> 00:08:16.040
it might be pointing
in this direction.

00:08:16.040 --> 00:08:18.960
Or in fact, it will be
pointing in this direction.

00:08:18.960 --> 00:08:21.190
I'm obviously not
drawing it that exact.

00:08:21.190 --> 00:08:24.590
And then the bottom of the
arrow will come out over here.

00:08:24.590 --> 00:08:27.810
So if you think about that,
if you wait 1,800 years,

00:08:27.810 --> 00:08:29.565
and once again, the
tilt hasn't changed

00:08:29.565 --> 00:08:32.549
or it's changed a little bit,
but what the precession has

00:08:32.549 --> 00:08:35.919
done, tracing out this circle
has changed the direction

00:08:35.919 --> 00:08:37.710
of this arrow, it
changed the direction

00:08:37.710 --> 00:08:39.770
of our axis of rotation.

00:08:39.770 --> 00:08:46.830
And if you wait 1,800 years when
will the Northern Hemisphere be

00:08:46.830 --> 00:08:49.250
pointed most away from the sun.

00:08:49.250 --> 00:08:51.670
Well, now, it won't
be pointed most away

00:08:51.670 --> 00:08:55.000
from the sun at this point
in space relative to the sun

00:08:55.000 --> 00:08:58.160
anymore because now
its axis of rotation

00:08:58.160 --> 00:09:01.710
looks something like this.

00:09:01.710 --> 00:09:05.090
So now if we wait-- or I
should say in 1,800 years,

00:09:05.090 --> 00:09:07.740
it will be most pointed away,
or the Northern Hemisphere

00:09:07.740 --> 00:09:11.650
will be most pointed away from
the sun, about a month earlier.

00:09:11.650 --> 00:09:13.220
So about a month earlier.

00:09:13.220 --> 00:09:17.780
It'll be most pointed away from
the sun about a month earlier.

00:09:17.780 --> 00:09:20.970
So this is when it will be
most pointed away from the sun.

00:09:23.500 --> 00:09:25.105
To today's time,
we would say, no,

00:09:25.105 --> 00:09:26.730
that's still not the
most pointed away,

00:09:26.730 --> 00:09:28.920
but since we have
this precession,

00:09:28.920 --> 00:09:32.280
since the direction of the
tilt I guess we could say,

00:09:32.280 --> 00:09:37.270
or the direction of our
rotational axis is changing,

00:09:37.270 --> 00:09:40.600
we are now at a different
point in our orbit

00:09:40.600 --> 00:09:43.267
where we are most pointed
away from the sun.

00:09:43.267 --> 00:09:45.100
So this is 1,800 years
later, approximately.

00:09:50.670 --> 00:09:53.010
So now based on
this, and I think

00:09:53.010 --> 00:09:56.040
this is what Vicksoma might have
been hinting at, you say, look,

00:09:56.040 --> 00:09:57.570
OK, it's earlier in our orbit.

00:09:57.570 --> 00:10:01.040
Wouldn't this now be November?

00:10:01.040 --> 00:10:02.100
And the answer is no.

00:10:02.100 --> 00:10:05.250
It will still be December 22nd.

00:10:05.250 --> 00:10:10.800
This will still be December 21st
or 22nd Depending on the year.

00:10:14.310 --> 00:10:17.130
Still be the same
date, and that's

00:10:17.130 --> 00:10:22.000
because our calendar is based
on when we are most tilted away

00:10:22.000 --> 00:10:24.960
or when we are most
tilted towards the sun.

00:10:24.960 --> 00:10:27.540
So by definition, this is
when we are most tilted away

00:10:27.540 --> 00:10:29.930
so this will be the
winter solstice.

00:10:29.930 --> 00:10:31.576
So what happens
is every year-- so

00:10:31.576 --> 00:10:32.950
the way I drew it
right over here

00:10:32.950 --> 00:10:34.450
and, actually, this
perihelion actually

00:10:34.450 --> 00:10:35.630
changes over time as well.

00:10:35.630 --> 00:10:37.910
There's a precession of
the perihelion as well,

00:10:37.910 --> 00:10:39.750
but I'm not going to
go into that right now.

00:10:39.750 --> 00:10:42.120
So if you fast
forward 1,800 years,

00:10:42.120 --> 00:10:44.220
all that's going to
happen is that what

00:10:44.220 --> 00:10:46.110
we consider by
our calendar to be

00:10:46.110 --> 00:10:50.240
December 22nd in an
absolute point in our orbit

00:10:50.240 --> 00:10:52.760
we'll be earlier in our
orbit, but we're still

00:10:52.760 --> 00:10:54.830
going to call it December 22nd.

00:10:54.830 --> 00:10:58.680
And so the perihelion is going
to be further away from that

00:10:58.680 --> 00:11:01.270
December 22nd, it's actually
going to be a month further

00:11:01.270 --> 00:11:04.170
away, so the perihelion
1,800 years from now

00:11:04.170 --> 00:11:09.440
won't be in January,
it will be in February.

00:11:09.440 --> 00:11:12.370
So the real takeaway
here is that our calendar

00:11:12.370 --> 00:11:17.210
is based not on the exact point
in space relative to the sun.

00:11:17.210 --> 00:11:21.790
Our calendar is based on the
maximum tilt towards or away

00:11:21.790 --> 00:11:24.000
from the sun, and
that, as we see,

00:11:24.000 --> 00:11:26.710
that is slightly
changing in terms

00:11:26.710 --> 00:11:29.670
of where it occurs in the
absolute point in space.

00:11:29.670 --> 00:11:33.070
I think it's changing by
roughly 20 minutes a year.

00:11:33.070 --> 00:11:37.290
So every year, the perihelion
is getting 20 minutes later.

00:11:37.290 --> 00:11:39.000
If we wanted to
use the perihelion,

00:11:39.000 --> 00:11:46.340
if we wanted to use the exact
point in space as our calendar,

00:11:46.340 --> 00:11:48.360
our year would actually
be about that-- I

00:11:48.360 --> 00:11:51.604
don't know what it is-- roughly
20 or 25 minutes longer,

00:11:51.604 --> 00:11:53.520
but it actually makes a
little much more sense

00:11:53.520 --> 00:11:55.470
to think about it from
the tilt because that's

00:11:55.470 --> 00:11:57.570
what dictates the seasons,
and that's actually

00:11:57.570 --> 00:11:59.630
what's most
observable from Earth,

00:11:59.630 --> 00:12:03.600
where the sun is in the sky.


Are southern hemisphere seasons more severe?

WEBVTT
Kind: captions
Language: en

00:00:00.600 --> 00:00:02.970
In previous videos we've
already talked about the idea

00:00:02.970 --> 00:00:04.990
that there are times
in Earth's orbit

00:00:04.990 --> 00:00:06.740
when it is closer
to the sun and when

00:00:06.740 --> 00:00:08.550
it is farther from the sun.

00:00:08.550 --> 00:00:09.926
And when it is
closer to the sun,

00:00:09.926 --> 00:00:12.300
so let's say that this is the
time in the orbit when it's

00:00:12.300 --> 00:00:14.060
closer to the sun,
this is the perihelion.

00:00:18.430 --> 00:00:20.710
And when it's
farthest from the sun,

00:00:20.710 --> 00:00:24.240
and I'm exaggerating the
difference, this is aphelion.

00:00:24.240 --> 00:00:27.164
This is the aphelion
in our orbit,

00:00:27.164 --> 00:00:28.580
when we are farthest
from the sun.

00:00:28.580 --> 00:00:32.355
And maybe our orbit looks
something like this.

00:00:36.320 --> 00:00:38.660
I shouldn't have the
bulge over there.

00:00:38.660 --> 00:00:43.040
So maybe our orbit looks
something like this over here.

00:00:43.040 --> 00:00:45.940
And what I point out in the
first video where we discuss

00:00:45.940 --> 00:00:49.390
this is that this is not
the cause of the seasons.

00:00:49.390 --> 00:00:53.872
Even though we are 3% closer
right now, the way our orbit

00:00:53.872 --> 00:00:55.580
is set up, and we'll
see in future videos

00:00:55.580 --> 00:00:59.600
that the difference or
the eccentricity, or how

00:00:59.600 --> 00:01:02.320
elliptical the orbit is,
does change over time,

00:01:02.320 --> 00:01:05.180
how much it deviates
from being circular.

00:01:05.180 --> 00:01:07.850
That's one way to think
about eccentricity.

00:01:07.850 --> 00:01:09.920
That does change over time.

00:01:09.920 --> 00:01:12.560
But right now when we
are closest to the sun

00:01:12.560 --> 00:01:15.640
we are 3% closer than when
we are farthest from the sun.

00:01:15.640 --> 00:01:19.654
So 3% closer than at aphelion.

00:01:19.654 --> 00:01:22.070
And we point out in the first
video when we discussed this

00:01:22.070 --> 00:01:24.610
that this is not the
cause of the seasons.

00:01:24.610 --> 00:01:27.810
And in particular, perihelion
, when we were closest

00:01:27.810 --> 00:01:31.320
to the sun, when we actually
have the most radiation from

00:01:31.320 --> 00:01:36.730
the sun, that's actually when
we have the Northern Hemisphere

00:01:36.730 --> 00:01:37.230
winter.

00:01:37.230 --> 00:01:38.820
So this occurs right over here.

00:01:38.820 --> 00:01:42.730
This occurs in January.

00:01:42.730 --> 00:01:45.030
And aphelion occurs in July.

00:01:48.150 --> 00:01:50.260
Now, based on this,
this might lead

00:01:50.260 --> 00:01:53.600
to an interesting question,
because let's think

00:01:53.600 --> 00:01:59.100
about January when
we're at perihelion,

00:01:59.100 --> 00:02:03.610
and let's think about July
when we're at aphelion.

00:02:03.610 --> 00:02:07.047
Let me draw a quick
globe right over here.

00:02:07.047 --> 00:02:08.380
And let's make that the equator.

00:02:08.380 --> 00:02:09.880
And I'll draw it
in both situations.

00:02:12.670 --> 00:02:15.510
So January is obviously when
we have the Northern Hemisphere

00:02:15.510 --> 00:02:16.010
winter.

00:02:16.010 --> 00:02:19.150
So I'll paint it in
blue right over here.

00:02:19.150 --> 00:02:20.070
It is winter.

00:02:22.780 --> 00:02:25.440
And July is when we have
the Northern Hemisphere

00:02:25.440 --> 00:02:28.640
summer, or the Southern
Hemisphere winter.

00:02:28.640 --> 00:02:33.370
So then we have
winter during July

00:02:33.370 --> 00:02:34.510
in the Southern Hemisphere.

00:02:34.510 --> 00:02:37.860
And let me put summer
in a more summery color.

00:02:37.860 --> 00:02:39.700
I guess that orange is
a pretty good color,

00:02:39.700 --> 00:02:41.960
but that's not orange.

00:02:41.960 --> 00:02:43.110
Here's orange.

00:02:43.110 --> 00:02:44.370
All right.

00:02:44.370 --> 00:02:45.830
That's orange.

00:02:45.830 --> 00:02:47.930
And that's orange.

00:02:47.930 --> 00:02:49.095
So these are summer.

00:02:52.270 --> 00:02:54.760
So that's the summer in the
Southern Hemisphere, which

00:02:54.760 --> 00:02:57.520
occurs during the winter the
Northern Hemisphere, and vice

00:02:57.520 --> 00:02:58.170
versa.

00:02:58.170 --> 00:02:59.680
Summer in the
Northern Hemisphere

00:02:59.680 --> 00:03:03.000
occurs during winter in
the Southern Hemisphere.

00:03:03.000 --> 00:03:05.260
And so the question might
be rising in your head,

00:03:05.260 --> 00:03:07.870
and I did see a few comments
on that first video asking

00:03:07.870 --> 00:03:09.600
this question, and
it's a good one.

00:03:09.600 --> 00:03:14.250
If we are closer to
the sun in January,

00:03:14.250 --> 00:03:16.950
or we are closest to
the sun in January--

00:03:16.950 --> 00:03:19.200
this is the perihelion
right over here--

00:03:19.200 --> 00:03:22.520
and so we are getting more
solar radiation in January,

00:03:22.520 --> 00:03:24.830
does that moderate the winter?

00:03:24.830 --> 00:03:30.862
Does that moderate the winter
in the Northern Hemisphere?

00:03:30.862 --> 00:03:32.570
Or I guess another
way to think about it,

00:03:32.570 --> 00:03:35.660
does it make the summer
in the Southern Hemisphere

00:03:35.660 --> 00:03:37.960
when we are closer to
the sun, does it make it

00:03:37.960 --> 00:03:42.570
more extreme, or hotter?

00:03:42.570 --> 00:03:47.380
And vice versa, in July, when
we are farthest from the sun,

00:03:47.380 --> 00:03:50.050
does that moderate the
Northern Hemisphere winter?

00:03:50.050 --> 00:03:51.620
Because it's hot
out there, but hey,

00:03:51.620 --> 00:03:53.203
we're little bit
farther from the sun.

00:03:53.203 --> 00:03:56.390
And does it make the Southern
Hemisphere winter colder?

00:03:56.390 --> 00:03:58.760
So once again does it
make this more extreme?

00:03:58.760 --> 00:04:02.370
Because it's already winter
and we're farther from the sun.

00:04:02.370 --> 00:04:04.652
So maybe we're also
getting less radiation.

00:04:04.652 --> 00:04:06.610
And so there's a couple
ways to think about it.

00:04:06.610 --> 00:04:09.000
One, it is true that
when we're farther

00:04:09.000 --> 00:04:12.480
we are getting a little bit
less radiation from the sun

00:04:12.480 --> 00:04:15.010
or we're getting heated
up a little bit less.

00:04:15.010 --> 00:04:18.860
But the one reality is that
the Southern Hemisphere climate

00:04:18.860 --> 00:04:24.080
as a whole is not more
extreme despite getting

00:04:24.080 --> 00:04:26.790
more solar energy in
the summer and getting

00:04:26.790 --> 00:04:28.670
less solar energy in the winter.

00:04:28.670 --> 00:04:30.530
And the reason why it
is not as extreme--

00:04:30.530 --> 00:04:31.904
Let me draw the
equator here just

00:04:31.904 --> 00:04:34.090
so we can separate
our hemispheres.

00:04:34.090 --> 00:04:37.700
The main reason it is believed
that it is not more extreme

00:04:37.700 --> 00:04:41.247
is that the Southern Hemisphere
has a lot more water in it.

00:04:41.247 --> 00:04:43.330
So just if you look at the
surface of the Southern

00:04:43.330 --> 00:04:45.400
Hemisphere you're looking
at a lot more water

00:04:45.400 --> 00:04:47.415
than the surface of the
Northern Hemisphere.

00:04:47.415 --> 00:04:49.540
And this is, of course,
it's a Mercator projection,

00:04:49.540 --> 00:04:51.831
and so it distorts things so
that things near the poles

00:04:51.831 --> 00:04:54.370
get really kind of built
up to look really huge even

00:04:54.370 --> 00:04:56.010
though they really
aren't that big.

00:04:56.010 --> 00:04:59.210
Greenland really isn't
larger than South America.

00:04:59.210 --> 00:05:03.150
It just spreads them out so
that you can kind of flatten out

00:05:03.150 --> 00:05:04.339
the map, so to speak.

00:05:04.339 --> 00:05:06.130
But the Southern
Hemisphere has more water,

00:05:06.130 --> 00:05:08.088
and as you may have
learned in chemistry class,

00:05:08.088 --> 00:05:11.670
water has a higher
specific heat.

00:05:11.670 --> 00:05:16.370
It takes a higher specific
energy, more heat,

00:05:16.370 --> 00:05:23.210
to raise water a degree than
it does to raise land a degree.

00:05:23.210 --> 00:05:25.810
And so water can
absorb more energy.

00:05:25.810 --> 00:05:28.160
Or when there's
less energy, water

00:05:28.160 --> 00:05:30.760
will release more energy
without dropping as much

00:05:30.760 --> 00:05:31.540
of a temperature.

00:05:31.540 --> 00:05:37.400
So water has a moderating
influence on the climate.

00:05:37.400 --> 00:05:39.860
So even though the summers
in the Southern Hemisphere

00:05:39.860 --> 00:05:41.700
actually are getting
more radiation

00:05:41.700 --> 00:05:44.090
than the summers in the
Northern Hemisphere,

00:05:44.090 --> 00:05:45.956
it's moderated on the
actual temperature

00:05:45.956 --> 00:05:47.330
because the water
has the ability

00:05:47.330 --> 00:05:50.270
to absorb more of that
heat without changing

00:05:50.270 --> 00:05:52.670
the temperature as dramatically.

00:05:52.670 --> 00:05:55.980
Now, with that said, it is
true that in general Antarctica

00:05:55.980 --> 00:05:57.730
is colder.

00:05:57.730 --> 00:06:01.830
Antarctica is colder
than the North Pole.

00:06:01.830 --> 00:06:03.520
But the main reason
why Antarctica

00:06:03.520 --> 00:06:05.680
is colder, besides the
fact that it's on land,

00:06:05.680 --> 00:06:09.920
as opposed to the
North Pole being

00:06:09.920 --> 00:06:11.530
in the center of
the Arctic Ocean,

00:06:11.530 --> 00:06:15.560
is that it's actually a huge,
very high altitude ice sheet.

00:06:15.560 --> 00:06:17.920
And so the altitude
for most of Antarctica

00:06:17.920 --> 00:06:19.730
is around 8,000 feet.

00:06:19.730 --> 00:06:21.760
So it's kind of like
an alpine altitude.

00:06:21.760 --> 00:06:23.380
So the main reason
why it's colder

00:06:23.380 --> 00:06:27.175
is possibly being farther
away from the sun in winter

00:06:27.175 --> 00:06:29.800
might play some role there, but
the main reason why it's colder

00:06:29.800 --> 00:06:32.030
is it is just at a
much higher altitude,

00:06:32.030 --> 00:06:34.390
and it's to some degree
insulated from the water,

00:06:34.390 --> 00:06:36.290
or I guess you could
say it's on the land.

00:06:36.290 --> 00:06:37.990
So especially during
the long winters

00:06:37.990 --> 00:06:39.830
it's going to get
that much colder.

00:06:39.830 --> 00:06:42.440
But I'll leave you there,
and to some degree,

00:06:42.440 --> 00:06:45.900
and this is the other aspect
of it, during the summers--

00:06:45.900 --> 00:06:48.440
And all of this stuff
is super complicated.

00:06:48.440 --> 00:06:50.274
So you can't just draw
out one rule of thumb

00:06:50.274 --> 00:06:51.398
and say this is the reason.

00:06:51.398 --> 00:06:52.850
But these are all
the influences--

00:06:52.850 --> 00:06:56.350
is that if you have a
super large ice sheet

00:06:56.350 --> 00:06:59.660
it's also more likely to
reflect more of the energy

00:06:59.660 --> 00:07:01.600
because it's white,
as opposed to a darker

00:07:01.600 --> 00:07:03.410
color like the
ocean or the land.

00:07:03.410 --> 00:07:05.410
And so you can think about
all of those factors,

00:07:05.410 --> 00:07:08.130
but the general answer
is it's a good question.

00:07:08.130 --> 00:07:10.450
But overall, the climate
in the Southern Hemisphere

00:07:10.450 --> 00:07:13.020
is not more extreme than
the climate in the Southern

00:07:13.020 --> 00:07:16.196
Hemisphere, even though
Antarctica is colder.


Milankovitch cycles precession and obliquity

WEBVTT
Kind: captions
Language: en

00:00:00.550 --> 00:00:02.490
We've learned in
previous videos that

00:00:02.490 --> 00:00:05.290
relative to the orbital
plane around the sun

00:00:05.290 --> 00:00:07.540
or the plane of Earth's
orbit around the sun,

00:00:07.540 --> 00:00:09.900
the Earth has a certain tilt.

00:00:09.900 --> 00:00:13.790
So let me draw the Earth's tilt
relative to that orbital plane

00:00:13.790 --> 00:00:16.129
right over here.

00:00:16.129 --> 00:00:19.090
So this if this is the
orbital plane right over here

00:00:19.090 --> 00:00:25.720
so we're looking right directly
sideways on this orbital plane

00:00:25.720 --> 00:00:28.110
that I've drawn in orange.

00:00:28.110 --> 00:00:30.460
And maybe at the point
in Earth's orbit right

00:00:30.460 --> 00:00:32.299
now, maybe the sun
is to the left,

00:00:32.299 --> 00:00:34.630
and so the rays from
the sun are coming

00:00:34.630 --> 00:00:36.600
in this general direction.

00:00:36.600 --> 00:00:41.220
We've learned that the
Earth as a certain tilt,

00:00:41.220 --> 00:00:43.760
and when I mean that
it means if you think

00:00:43.760 --> 00:00:48.280
about the axis around which
it's rotating it's not straight

00:00:48.280 --> 00:00:51.586
up from the orbital
plane, it is at an angle.

00:00:51.586 --> 00:00:52.460
And let me draw that.

00:00:52.460 --> 00:00:54.960
So if I were to draw an arrow
that's coming out of the North

00:00:54.960 --> 00:00:56.650
Pole it would look like that.

00:00:56.650 --> 00:00:59.830
And maybe I'll draw an arrow
coming out of the South Pole.

00:00:59.830 --> 00:01:04.019
And the Earth is rotating in
that direction right over here,

00:01:04.019 --> 00:01:08.210
and you notice this axis that
I've drawn this arrow on,

00:01:08.210 --> 00:01:10.410
it is not straight up and down.

00:01:10.410 --> 00:01:22.980
And right now it is at
an angle of 23.4 degrees

00:01:22.980 --> 00:01:27.180
with the vertical, with
being straight up and down.

00:01:27.180 --> 00:01:28.920
And we've learned
how this is what

00:01:28.920 --> 00:01:32.930
is the primary
cause of our seasons

00:01:32.930 --> 00:01:35.380
in that when the Northern
Hemisphere is pointed

00:01:35.380 --> 00:01:38.000
towards the sun it's getting
a disproportionate amount

00:01:38.000 --> 00:01:39.540
of the solar radiation.

00:01:39.540 --> 00:01:41.780
Whatever's going through
the atmosphere has

00:01:41.780 --> 00:01:44.530
to go through less atmosphere
and the things in the Northern

00:01:44.530 --> 00:01:46.400
Hemisphere are
getting more daylight.

00:01:46.400 --> 00:01:50.140
And when the Earth is on
the other side of the sun

00:01:50.140 --> 00:01:52.360
and the Northern
Hemisphere is pointed away

00:01:52.360 --> 00:01:54.990
from the sun then the
opposite is going to happen.

00:01:54.990 --> 00:01:57.180
And the reverse is true for
the Southern Hemisphere.

00:01:57.180 --> 00:01:59.260
But in that video
when we talk about how

00:01:59.260 --> 00:02:03.700
tilt can affect
the seasons, I also

00:02:03.700 --> 00:02:07.280
kind of hinted a little bit
that this is the current tilt

00:02:07.280 --> 00:02:10.120
right now, and over
long periods of time

00:02:10.120 --> 00:02:11.930
that this tilt will change.

00:02:11.930 --> 00:02:15.710
And in particular, it will
vary, and even the boundaries

00:02:15.710 --> 00:02:19.760
for this varying are different
for the past million years

00:02:19.760 --> 00:02:21.720
than they will be for
the next million years,

00:02:21.720 --> 00:02:29.780
but it varies roughly between
22.1 degrees and 24.5 degrees.

00:02:29.780 --> 00:02:32.350
And just to make it
clear that it's not

00:02:32.350 --> 00:02:34.120
wobbling back and
forth like this,

00:02:34.120 --> 00:02:37.660
and just to visualize
22.1 versus 24.5,

00:02:37.660 --> 00:02:39.380
it's not a huge difference.

00:02:39.380 --> 00:02:42.640
So if this is 23.4-- and
I'm not measuring exactly--

00:02:42.640 --> 00:02:50.139
maybe pointing in this
direction, maybe 22.1

00:02:50.139 --> 00:02:51.430
would look something like that.

00:02:51.430 --> 00:02:53.030
In fact, I've exaggerated it.

00:02:53.030 --> 00:02:56.770
And maybe 24.5 would
look something like that.

00:02:56.770 --> 00:02:58.500
And so it's not a
huge difference,

00:02:58.500 --> 00:03:01.390
but it is enough of a difference
so we believe to actually

00:03:01.390 --> 00:03:05.810
have a significant impact
on what the climate is like

00:03:05.810 --> 00:03:07.620
or what the seasons
are like, especially

00:03:07.620 --> 00:03:14.400
in terms of how much of a chance
different parts of our planet

00:03:14.400 --> 00:03:16.542
have a chance to freeze
over or not freeze over

00:03:16.542 --> 00:03:18.500
and all the rest, or how
much sunlight they get

00:03:18.500 --> 00:03:19.230
and all the rest.

00:03:19.230 --> 00:03:21.290
So it has some
impact, but I want

00:03:21.290 --> 00:03:24.360
to make it clear that it
takes a long period of time.

00:03:24.360 --> 00:03:27.680
That it actually
takes 41,000 years

00:03:27.680 --> 00:03:30.470
to go from a minimum
tilt to a maximum tilt

00:03:30.470 --> 00:03:32.600
and then back to a minimum tilt.

00:03:32.600 --> 00:03:35.510
41,000 years.

00:03:35.510 --> 00:03:38.730
And right now at a
tilt of 23.4 degrees,

00:03:38.730 --> 00:03:41.200
we're someplace right
smack in between.

00:03:41.200 --> 00:03:49.720
And we think the last
maximum was at 8,700 BCE,

00:03:49.720 --> 00:03:52.630
before the common era, or
you could say before Christ.

00:03:52.630 --> 00:03:55.870
And that the next minimum, when
our tilt has been minimized,

00:03:55.870 --> 00:03:57.900
the next time our
tilt will be minimized

00:03:57.900 --> 00:04:04.350
will be in the year 11,800.

00:04:04.350 --> 00:04:06.480
So this isn't something
that's happening overnight,

00:04:06.480 --> 00:04:08.160
but it is something
that could affect

00:04:08.160 --> 00:04:10.760
our climate over
long periods of time.

00:04:10.760 --> 00:04:12.910
And this is just one
factor, and sometimes

00:04:12.910 --> 00:04:15.720
this changing of the tilt,
a fancier word for tilt

00:04:15.720 --> 00:04:19.810
is sometimes given,
is obliquity,

00:04:19.810 --> 00:04:22.350
but this is really just
a fancy word for tilt.

00:04:22.350 --> 00:04:25.670
This changing of the obliquity--
or the changing of the tilt--

00:04:25.670 --> 00:04:29.430
is one of these changes in
Earth's rotation or Earth's

00:04:29.430 --> 00:04:33.960
orbit around the sun that
might have long-term cycles

00:04:33.960 --> 00:04:36.490
or effects on Earth's
climate, and maybe they

00:04:36.490 --> 00:04:40.490
do help cause certain ice
ages when they act together

00:04:40.490 --> 00:04:42.490
with each other
over certain cycles.

00:04:42.490 --> 00:04:45.200
And broadly, this
entire class of cycles

00:04:45.200 --> 00:04:47.880
are called Milankovitch cycles.

00:04:47.880 --> 00:04:50.610
Milankovitch, he was
a Serbian scientist,

00:04:50.610 --> 00:04:54.320
who's the guy who theorized that
these changes in Earth's orbit

00:04:54.320 --> 00:04:57.160
might be responsible
for long-term climate

00:04:57.160 --> 00:04:59.850
change or maybe some cycles
where we enter ice ages

00:04:59.850 --> 00:05:01.280
and get out of ice
ages or we have

00:05:01.280 --> 00:05:03.520
more extreme or less
extreme weather.

00:05:03.520 --> 00:05:09.550
So these are
Milankovitch cycles.

00:05:09.550 --> 00:05:12.020
And changes in the tilt,
or the elbow obliquity,

00:05:12.020 --> 00:05:14.560
are just one of the
possible factors

00:05:14.560 --> 00:05:16.430
playing into
Milankovitch cycles.

00:05:16.430 --> 00:05:18.030
And what I want to do in this
video and in the next video

00:05:18.030 --> 00:05:19.821
is talk about all of
the different factors,

00:05:19.821 --> 00:05:22.730
or at least summarize all
of the different factors.

00:05:22.730 --> 00:05:23.510
Now, another one.

00:05:23.510 --> 00:05:26.780
This one is pretty intuitive for
me that this tilt can change.

00:05:26.780 --> 00:05:29.250
One that's a little bit less
intuitive when you first

00:05:29.250 --> 00:05:32.510
think about it is something
called precession.

00:05:32.510 --> 00:05:38.800
And the idea behind
precession, I

00:05:38.800 --> 00:05:41.350
guess the best analogy
I can think of,

00:05:41.350 --> 00:05:44.080
is if you imagine
a top, or maybe you

00:05:44.080 --> 00:05:46.720
could imagine Earth as
a top right over here.

00:05:46.720 --> 00:05:50.910
The top is spinning, the top
is spinning in this direction,

00:05:50.910 --> 00:05:53.730
and obliquity tells
you, essentially,

00:05:53.730 --> 00:05:55.897
how much it's wobbling.

00:05:55.897 --> 00:05:57.480
Actually, let me
think of it this way.

00:05:57.480 --> 00:05:59.280
Imagine a wobbling top.

00:05:59.280 --> 00:06:01.740
So it's rotating like
this, it's tilted,

00:06:01.740 --> 00:06:06.230
and then it is
also if you imagine

00:06:06.230 --> 00:06:08.980
that this was a pole up here
that's coming out of the pole,

00:06:08.980 --> 00:06:10.646
that this was actually
a physical arrow,

00:06:10.646 --> 00:06:16.320
that that arrow itself
would be rotating.

00:06:16.320 --> 00:06:19.050
So the best way to think
about it is a wobbling top.

00:06:19.050 --> 00:06:21.560
If you think after
some point of time

00:06:21.560 --> 00:06:26.790
this thing would wobble so
it would look like this.

00:06:26.790 --> 00:06:28.510
So now the arrow is
pointing that way.

00:06:28.510 --> 00:06:30.093
And if you wait a
few more seconds now

00:06:30.093 --> 00:06:34.802
maybe the arrow is pointing
a little bit out of the page.

00:06:34.802 --> 00:06:36.510
And then you wait a
few more seconds then

00:06:36.510 --> 00:06:39.080
it's pointing in this direction,
it's pointing into the page.

00:06:39.080 --> 00:06:41.950
And so this whole time, the
obliquity isn't changing.

00:06:41.950 --> 00:06:47.730
The obliquity you can view
it as how far is that wobble.

00:06:47.730 --> 00:06:50.620
You could imagine how far
from vertical is that wobble,

00:06:50.620 --> 00:06:53.290
and no matter where we
are in that rotation

00:06:53.290 --> 00:06:56.320
it hasn't changed, and you
could imagine the precession

00:06:56.320 --> 00:06:59.195
as where we are in the wobble.

00:07:01.799 --> 00:07:04.090
And I want to-- this is a
little bit hard to visualize,

00:07:04.090 --> 00:07:06.410
and hopefully, as we think
about it in different ways

00:07:06.410 --> 00:07:07.700
and I draw different
diagrams it'll

00:07:07.700 --> 00:07:10.170
become a little bit clearer--
but I want to make it clear.

00:07:10.170 --> 00:07:15.280
Just as it takes a long
time for the inclination

00:07:15.280 --> 00:07:18.410
to change from a minimum value
to a maximum value and back,

00:07:18.410 --> 00:07:22.130
it takes a huge amount of
time for Earth's precession

00:07:22.130 --> 00:07:24.030
to change in a significant way.

00:07:24.030 --> 00:07:29.020
So for this top to-- if you
imagined this arrow popping

00:07:29.020 --> 00:07:32.230
out, for this arrow to actually
trace out an entire loop,

00:07:32.230 --> 00:07:35.970
it takes 26,000 years.

00:07:35.970 --> 00:07:40.770
So 26,000 years to have an
entire cycle of precession.

00:07:40.770 --> 00:07:42.400
Now, what I want to
do is think about

00:07:42.400 --> 00:07:44.950
given that this
precession is occurring,

00:07:44.950 --> 00:07:48.020
I want to think about how
that would affect our seasons,

00:07:48.020 --> 00:07:49.810
or how it would
actually affect how

00:07:49.810 --> 00:07:52.700
we think about the
year or the calendar.

00:07:52.700 --> 00:07:57.620
So let's draw the orbit
of Earth around the sun.

00:07:57.620 --> 00:08:00.610
So here is my sun
right over here.

00:08:00.610 --> 00:08:02.329
And here is the orbit of Earth.

00:08:02.329 --> 00:08:03.870
And I'm not going
to think too much--

00:08:03.870 --> 00:08:05.270
I'm going to assume
that it's almost

00:08:05.270 --> 00:08:06.660
circular for the
sake of this video.

00:08:06.660 --> 00:08:08.243
In future videos,
we'll talk about how

00:08:08.243 --> 00:08:12.090
the eccentricity-- or how
elliptical the orbit is--

00:08:12.090 --> 00:08:14.440
can also affect the
Malinkovitch cycles

00:08:14.440 --> 00:08:16.550
or play into the
Malinkovitch cycles.

00:08:16.550 --> 00:08:23.630
But let's just draw the orbit of
Earth around the sun over here.

00:08:23.630 --> 00:08:27.530
And so you could imagine this
is, at one point in time,

00:08:27.530 --> 00:08:28.910
this is the Earth.

00:08:28.910 --> 00:08:33.789
Let's say it is tilted
towards the sun right now.

00:08:37.070 --> 00:08:39.900
And so is in the
Northern Hemisphere--

00:08:39.900 --> 00:08:41.669
and I'm assuming this
arrow is coming out

00:08:41.669 --> 00:08:44.169
of the North Pole-- this would
be the summer in the Northern

00:08:44.169 --> 00:08:44.980
Hemisphere.

00:08:44.980 --> 00:08:47.680
And then if you had no
precession, absolutely

00:08:47.680 --> 00:08:50.120
no precession, when you
go to this time of year

00:08:50.120 --> 00:08:54.680
you still have the
same direction of tilt.

00:08:54.680 --> 00:08:56.440
Let me do that in blue.

00:08:56.440 --> 00:08:59.630
You still have the
same direction of tilt.

00:08:59.630 --> 00:09:01.950
We're still pointing to the
same part of the universe.

00:09:01.950 --> 00:09:05.720
We still have the
same North Star.

00:09:05.720 --> 00:09:08.600
You go to this time, we're still
tilting in the same direction

00:09:08.600 --> 00:09:10.225
relative to the
universe, but we're not

00:09:10.225 --> 00:09:11.660
tilting away from the sun.

00:09:11.660 --> 00:09:15.056
And now this would be the winter
in the Northern Hemisphere.

00:09:15.056 --> 00:09:16.180
And we'd keep going around.

00:09:16.180 --> 00:09:18.138
And if you had no
precession, when you get back

00:09:18.138 --> 00:09:20.410
to this point over
here, we'd be tilting

00:09:20.410 --> 00:09:22.180
in the exact same direction.

00:09:22.180 --> 00:09:27.030
If your obliquity or if your
tilt changed a little bit,

00:09:27.030 --> 00:09:30.090
you might move up or
down, away, or towards

00:09:30.090 --> 00:09:33.134
the sun a little bit, but this
is all assuming no precession.

00:09:33.134 --> 00:09:34.550
Now, I'm going to
think about what

00:09:34.550 --> 00:09:36.800
happens if you do
have precession.

00:09:36.800 --> 00:09:39.320
So what's happening
with precession

00:09:39.320 --> 00:09:42.530
is when you go around
one time around the sun,

00:09:42.530 --> 00:09:44.780
by the time you get
to this point again

00:09:44.780 --> 00:09:47.730
you're not pointing at
exactly the same direction.

00:09:47.730 --> 00:09:50.160
You are now pointing
a little bit further

00:09:50.160 --> 00:09:55.030
so this arrow-- let me draw
it a little bit bigger.

00:09:55.030 --> 00:09:58.640
So this is the Earth
and this is that arrow.

00:09:58.640 --> 00:10:01.060
And this is hard to visualize
or at least it's hard for me

00:10:01.060 --> 00:10:02.280
to visualize.

00:10:02.280 --> 00:10:04.310
Well, once you get it,
it's easier to visualize,

00:10:04.310 --> 00:10:05.950
but the first time I
tried to understand it,

00:10:05.950 --> 00:10:08.070
it was hard for me to
understand how precession

00:10:08.070 --> 00:10:11.230
was different than obliquity
or different than tilt.

00:10:11.230 --> 00:10:15.640
Obliquity is how much
we're going from vertical.

00:10:15.640 --> 00:10:17.570
And so if we had
no precession, we

00:10:17.570 --> 00:10:21.260
would be exactly pointing in
that same direction every year.

00:10:21.260 --> 00:10:26.110
Now, with just precession alone
what happens is every year

00:10:26.110 --> 00:10:31.960
this arrow is slowly tracing out
a circle that goes like this.

00:10:31.960 --> 00:10:33.770
So I'm going to
exaggerate how much it's

00:10:33.770 --> 00:10:35.700
happening just so that
you can visualize it.

00:10:35.700 --> 00:10:38.120
So maybe after several
years that arrow

00:10:38.120 --> 00:10:41.810
is not-- when you're at that
same point relative to the sun,

00:10:41.810 --> 00:10:44.350
that same point in
the solar system,

00:10:44.350 --> 00:10:46.710
that arrow is no longer
pointing in that direction.

00:10:46.710 --> 00:10:49.320
It is now traced out a
little bit of that circle.

00:10:49.320 --> 00:10:52.745
So it is now pointing
in this direction.

00:10:55.920 --> 00:10:58.080
So if it is now pointing
in this direction,

00:10:58.080 --> 00:11:00.530
will that same point
in the solar system,

00:11:00.530 --> 00:11:04.820
that same point relative to
the sun, that same exact point

00:11:04.820 --> 00:11:08.350
in the orbit, will it
still be the summer

00:11:08.350 --> 00:11:09.970
in the Northern Hemisphere?

00:11:09.970 --> 00:11:13.140
Well, it won't because we're
now not pointing directly

00:11:13.140 --> 00:11:16.220
or we're not most inclined
to the sun at that point.

00:11:16.220 --> 00:11:17.880
Now, we would have
been most inclined

00:11:17.880 --> 00:11:20.750
to the sun a little bit earlier
in the year or a little bit

00:11:20.750 --> 00:11:22.590
earlier in the orbit.

00:11:22.590 --> 00:11:26.210
So we would have been
most inclined to the sun

00:11:26.210 --> 00:11:27.175
maybe over here.

00:11:29.890 --> 00:11:32.840
And it would take many,
many, many actual thousands

00:11:32.840 --> 00:11:37.950
of years for the precession
to change this much,

00:11:37.950 --> 00:11:43.080
but then over here this is where
when at this point in that year

00:11:43.080 --> 00:11:46.100
when we would be pointed
most towards the sun.

00:11:46.100 --> 00:11:49.040
So what the real
effect of precession

00:11:49.040 --> 00:11:51.680
is doing to our
seasons and doing

00:11:51.680 --> 00:11:55.370
to what our sense of what our
year is, is that every year

00:11:55.370 --> 00:11:58.430
relative to our orbit
on Earth, because Earth

00:11:58.430 --> 00:12:01.810
is kind of a top
that's slowly circling,

00:12:01.810 --> 00:12:03.780
that's slowly tracing
out this circle

00:12:03.780 --> 00:12:06.440
with, I guess you could
say, with its pole.

00:12:06.440 --> 00:12:09.370
What it's doing
is it's making it

00:12:09.370 --> 00:12:11.880
tilt towards the sun
or away from the sun

00:12:11.880 --> 00:12:14.709
a little bit earlier each year.

00:12:14.709 --> 00:12:16.750
I know it's hard to
visualize, but you could even

00:12:16.750 --> 00:12:19.670
take a top out and have
a basketball as the sun,

00:12:19.670 --> 00:12:22.390
and if you play with it,
you'll see how that works.

00:12:22.390 --> 00:12:26.670
And precession is another
one of those factors that

00:12:26.670 --> 00:12:29.800
play into, I should say,
the Malinkovitch cycles.

00:12:29.800 --> 00:12:32.130
And what we'll see is when
you combine precession,

00:12:32.130 --> 00:12:34.720
when you combine-- or I should
say change in precession, when

00:12:34.720 --> 00:12:37.090
you combine that
with changes in tilt,

00:12:37.090 --> 00:12:40.590
and you combine that with
changes in actual how circular

00:12:40.590 --> 00:12:44.360
or how elliptical the actual
orbit is and how that changes,

00:12:44.360 --> 00:12:48.810
then you might have a
respectable way of explaining,

00:12:48.810 --> 00:12:51.250
or some of explaining,
why Earth is entered

00:12:51.250 --> 00:12:54.790
into these climactic cycles
over many tens of thousands

00:12:54.790 --> 00:12:56.460
of years.


How earth's tilt causes seasons

WEBVTT
Kind: captions
Language: en

00:00:00.530 --> 00:00:03.440
In the last video, we talk
about how seasons on Earth

00:00:03.440 --> 00:00:06.550
are not caused by
how close Earth

00:00:06.550 --> 00:00:08.080
is to the sun in its orbit.

00:00:08.080 --> 00:00:10.460
And we also hint at the
fact that it's actually

00:00:10.460 --> 00:00:12.710
caused by the tilt of the earth.

00:00:12.710 --> 00:00:14.590
And so in this video,
I want to show you

00:00:14.590 --> 00:00:19.110
how the tilt of the earth
causes the seasons to happen.

00:00:19.110 --> 00:00:21.410
So let's draw-- so
I'm going to try

00:00:21.410 --> 00:00:23.634
to draw as many diagrams
as possible here.

00:00:23.634 --> 00:00:25.050
Because at least
for my brain they

00:00:25.050 --> 00:00:27.610
help me visualize what's
actually going on.

00:00:27.610 --> 00:00:30.450
So we could imagine
a top view first.

00:00:30.450 --> 00:00:32.479
So let's have a top view.

00:00:32.479 --> 00:00:35.120
That is the sun
right over there.

00:00:35.120 --> 00:00:37.430
And let me draw
the earth's orbit.

00:00:37.430 --> 00:00:41.510
So Earth's orbit maybe
looks something like that.

00:00:41.510 --> 00:00:43.712
Let me draw it almost,
it is almost circular.

00:00:43.712 --> 00:00:45.170
So I'll draw it as
something that's

00:00:45.170 --> 00:00:47.520
pretty close to a
circle right over here.

00:00:47.520 --> 00:00:50.350
And I'm going to draw Earth at
different points in its orbit.

00:00:50.350 --> 00:00:53.040
And I'm going to try
to depict the tilt

00:00:53.040 --> 00:00:55.470
of its rotational axis.

00:00:55.470 --> 00:00:58.980
And obviously, this is not drawn
anywhere near close to scale.

00:00:58.980 --> 00:01:00.880
Earth is much further
away from the sun,

00:01:00.880 --> 00:01:04.170
and much, much smaller
than the sun as well.

00:01:04.170 --> 00:01:05.940
So I'll draw the
earth at that point.

00:01:05.940 --> 00:01:07.315
And at this point,
the earth will

00:01:07.315 --> 00:01:09.620
be tilted away from the sun.

00:01:09.620 --> 00:01:12.020
So Earth's tilt does
not change if you

00:01:12.020 --> 00:01:13.670
think about the
direction, or at least

00:01:13.670 --> 00:01:15.253
over the course of
a year, if we think

00:01:15.253 --> 00:01:17.290
about relatively
small periods of time.

00:01:17.290 --> 00:01:19.720
It does not change
relative to the direction

00:01:19.720 --> 00:01:22.340
that it's pointing
at in the universe.

00:01:22.340 --> 00:01:24.490
And we'll talk about
that in a second.

00:01:24.490 --> 00:01:28.590
So let's say right over here we
are pointed away from the sun.

00:01:28.590 --> 00:01:32.820
So we're up and
out of this page.

00:01:32.820 --> 00:01:35.010
So if I wanted to put some
perspective on an arrow

00:01:35.010 --> 00:01:39.390
it would be up and-- actually
it would be more like

00:01:39.390 --> 00:01:42.060
up and out of this page.

00:01:42.060 --> 00:01:43.120
So that's the direction.

00:01:43.120 --> 00:01:44.680
If you were to come straight
out of the North Pole.

00:01:44.680 --> 00:01:46.846
And if you were to go
straight out of the South Pole

00:01:46.846 --> 00:01:49.021
you'd go below that
circle right over there.

00:01:49.021 --> 00:01:50.770
And if I wanted to
draw the same position,

00:01:50.770 --> 00:01:53.890
but if we're looking
sideways along the plane,

00:01:53.890 --> 00:01:58.234
the orbital plane, or the
plane of Earth's orbit.

00:01:58.234 --> 00:02:00.150
So if we're looking at
it from that direction.

00:02:00.150 --> 00:02:02.020
So let me do it this way.

00:02:02.020 --> 00:02:06.220
If we're looking at
directly sideways,

00:02:06.220 --> 00:02:12.590
this is the sun right
over here, and this

00:02:12.590 --> 00:02:14.085
is Earth at that position.

00:02:18.440 --> 00:02:20.100
This is Earth right over there.

00:02:20.100 --> 00:02:24.320
If I were to draw an arrow
pointing straight out

00:02:24.320 --> 00:02:31.520
of the North Pole it would
look something like this.

00:02:31.520 --> 00:02:32.895
So this arrow and
this arrow they

00:02:32.895 --> 00:02:35.450
are both popping straight
out of the North Pole.

00:02:35.450 --> 00:02:37.690
And so when we talk about
the tilt of the earth,

00:02:37.690 --> 00:02:40.550
we're talking about the
tilt of its orbital axis,

00:02:40.550 --> 00:02:43.020
kind of this pole that could
go straight between the South

00:02:43.020 --> 00:02:44.030
Pole and the North Pole.

00:02:44.030 --> 00:02:46.980
The angle between that and
a pole that would actually

00:02:46.980 --> 00:02:49.740
be at a 90-degree
angle, or perpendicular,

00:02:49.740 --> 00:02:51.890
to the plane of its orbit.

00:02:51.890 --> 00:02:55.130
And so compared to if it was
just straight up and down,

00:02:55.130 --> 00:02:57.170
relative to the
plane of the orbit.

00:02:57.170 --> 00:03:00.220
So this right here is the
angle of Earth's tilt.

00:03:00.220 --> 00:03:02.350
Let me draw that a
little bit bigger

00:03:02.350 --> 00:03:04.750
just so it becomes a
little bit clearer.

00:03:04.750 --> 00:03:07.280
So if this is the
plane of the orbit,

00:03:07.280 --> 00:03:10.340
we're looking sideways along
the plane of the orbit.

00:03:10.340 --> 00:03:12.170
And this is Earth
right over here.

00:03:15.440 --> 00:03:17.900
My best attempt
to draw a circle.

00:03:17.900 --> 00:03:19.330
That is Earth.

00:03:19.330 --> 00:03:20.790
Earth does not rotate.

00:03:20.790 --> 00:03:24.230
Its axis of rotation is not
perpendicular to the plane

00:03:24.230 --> 00:03:25.210
of the orbit.

00:03:25.210 --> 00:03:27.320
So this is how
Earth would orbit.

00:03:27.320 --> 00:03:30.080
This is how Earth
would rotate if it was.

00:03:30.080 --> 00:03:33.880
Earth rotates, Earth's
rotational axis

00:03:33.880 --> 00:03:36.950
is at an angle to
that vertical relative

00:03:36.950 --> 00:03:39.270
to the plane of its orbit,
I guess you could say it.

00:03:39.270 --> 00:03:42.097
It rotates at an
angle like this.

00:03:42.097 --> 00:03:43.430
So this would be the North Pole.

00:03:43.430 --> 00:03:44.750
That is the South Pole.

00:03:47.310 --> 00:03:52.540
And so it rotates like this.

00:03:52.540 --> 00:03:58.370
And that angle relative to
being vertical with respect

00:03:58.370 --> 00:04:02.670
to the orbital plane, this
angle right here for Earth

00:04:02.670 --> 00:04:14.770
right now is 23.4 degrees.

00:04:14.770 --> 00:04:17.029
And if we're talking about
relatively short periods

00:04:17.029 --> 00:04:20.320
of time, like our
lifespans, that is constant.

00:04:20.320 --> 00:04:24.080
But it is actually changing
over long periods of time.

00:04:24.080 --> 00:04:27.280
That is changing between--
and these are rough numbers--

00:04:27.280 --> 00:04:36.980
it is changing between 22.1
degrees and 24.5 degrees,

00:04:36.980 --> 00:04:38.810
if my sources are correct.

00:04:38.810 --> 00:04:41.394
But that gives a rough estimate
of what it's changing between.

00:04:41.394 --> 00:04:42.768
But I want to make
it clear, this

00:04:42.768 --> 00:04:44.150
is not happening overnight.

00:04:44.150 --> 00:04:47.810
The period to go from roughly
a 22-degree angle to a 24

00:04:47.810 --> 00:04:52.640
and 1/2-degree angle and back
to a 22-degree angle is 41,000

00:04:52.640 --> 00:04:53.140
years.

00:04:58.110 --> 00:05:01.320
And this long-term
change in the tilt, this

00:05:01.320 --> 00:05:04.850
might play into some of the
long-term climactic change.

00:05:04.850 --> 00:05:07.450
Maybe it might
contribute, on some level,

00:05:07.450 --> 00:05:11.110
to some of the ice ages that
have formed over Earth's past.

00:05:11.110 --> 00:05:13.470
But for the sake of thinking
about our annual seasons

00:05:13.470 --> 00:05:14.930
you don't have to
worry too much,

00:05:14.930 --> 00:05:16.596
or you don't have to
worry at all really

00:05:16.596 --> 00:05:17.560
about this variation.

00:05:17.560 --> 00:05:19.770
You really just have to
know that it is tilted.

00:05:19.770 --> 00:05:24.550
And right now it is tilted
at an angle of 23.4 degrees.

00:05:24.550 --> 00:05:27.310
Now you might say OK, I
understand what the tilt is.

00:05:27.310 --> 00:05:29.510
But how does that
change the seasons

00:05:29.510 --> 00:05:33.000
in either the Northern or
that Southern Hemisphere?

00:05:33.000 --> 00:05:36.930
And to do that, I'm going
to imagine the earth when

00:05:36.930 --> 00:05:40.050
the Northern Hemisphere is
most tilted away from the sun,

00:05:40.050 --> 00:05:42.540
and when it is most
tilted towards the sun.

00:05:42.540 --> 00:05:45.200
So remember this tilt, the
direction this arrow points

00:05:45.200 --> 00:05:47.580
into relative to the
rest of the universe,

00:05:47.580 --> 00:05:50.120
if we assume that
this tilt is at 23.4%,

00:05:50.120 --> 00:05:52.180
it's not changing
throughout the year.

00:05:52.180 --> 00:05:54.190
But depending on where
it is in the orbit

00:05:54.190 --> 00:05:56.740
it's either going to be
tilting away from the sun,

00:05:56.740 --> 00:05:58.840
as it is in this
example right over here.

00:05:58.840 --> 00:06:03.540
Or it will be tilting
towards the sun.

00:06:03.540 --> 00:06:06.770
I'll do the towards the
sun in this magenta color,

00:06:06.770 --> 00:06:08.420
or it would be tilting
towards the sun.

00:06:08.420 --> 00:06:10.940
So six months later when
the earth is over here,

00:06:10.940 --> 00:06:14.670
it's going to, relative to
the rest of the universe,

00:06:14.670 --> 00:06:21.020
it will be tilted in that same
direction, up out of this page

00:06:21.020 --> 00:06:21.920
and to the right.

00:06:26.290 --> 00:06:27.830
Just like it was over here.

00:06:27.830 --> 00:06:30.220
But now that it's on the
other side of the sun that

00:06:30.220 --> 00:06:32.450
makes it tilt a little
bit more towards the sun.

00:06:32.450 --> 00:06:36.620
If I were to draw
it right over here,

00:06:36.620 --> 00:06:42.910
it is now tilted
towards the sun.

00:06:42.910 --> 00:06:47.470
And what I want to think
about is how much sunlight

00:06:47.470 --> 00:06:49.670
will different parts
of the planet receive.

00:06:49.670 --> 00:06:51.420
And I'll focus on the
Northern Hemisphere.

00:06:51.420 --> 00:06:53.330
But you can make
a similar argument

00:06:53.330 --> 00:06:54.660
for the Southern Hemisphere.

00:06:54.660 --> 00:06:57.620
I want to think about how much
sunlight they receive when it's

00:06:57.620 --> 00:07:00.840
tilted away or tilted
towards the sun.

00:07:00.840 --> 00:07:03.459
And so let's think about
those two situations.

00:07:03.459 --> 00:07:05.500
So first of all, let's
think about this situation

00:07:05.500 --> 00:07:07.890
here where we are tilted
away from the sun.

00:07:07.890 --> 00:07:10.590
So let me zoom in a little bit.

00:07:10.590 --> 00:07:15.090
So this is the situation, where
we're tilted away from the sun.

00:07:15.090 --> 00:07:18.640
So if this is the vertical,
so let me draw it.

00:07:18.640 --> 00:07:20.310
I could actually just
use this diagram.

00:07:20.310 --> 00:07:21.770
But let me make it.

00:07:21.770 --> 00:07:24.305
So we're tilted away
from the sun like this.

00:07:24.305 --> 00:07:26.520
I'm going to do this
in a different color.

00:07:26.520 --> 00:07:29.020
So if we have an arrow coming
straight out of the North Pole

00:07:29.020 --> 00:07:30.020
it would look like this.

00:07:33.690 --> 00:07:37.420
And we are rotating
around like that.

00:07:37.420 --> 00:07:40.384
So we're out of the page
on the left-hand side,

00:07:40.384 --> 00:07:42.300
and then into the page
on the right-hand side.

00:07:42.300 --> 00:07:45.200
And so we're rotating
towards the east, constantly.

00:07:45.200 --> 00:07:50.380
So this arrow is in the
direction of the east.

00:07:50.380 --> 00:07:52.920
So when we're at this point
in Earth's orbit, and actually

00:07:52.920 --> 00:07:54.090
let me copy and paste this.

00:07:54.090 --> 00:07:56.600
And I'm going to use
the same exact diagram

00:07:56.600 --> 00:07:58.970
for the different seasons.

00:07:58.970 --> 00:08:01.410
So let me copy.

00:08:01.410 --> 00:08:03.529
And then let me paste
this exact diagram.

00:08:03.529 --> 00:08:05.445
I'll do it over here for
two different points.

00:08:09.430 --> 00:08:13.440
So when we are here
in Earth's orbit

00:08:13.440 --> 00:08:14.857
where is the
sunlight coming from?

00:08:14.857 --> 00:08:17.398
Well, it's going to be coming
from the left, at least the way

00:08:17.398 --> 00:08:19.120
I've drawn the diagram
right over here.

00:08:19.120 --> 00:08:29.050
So the sunlight is coming from
the left in this situation.

00:08:29.050 --> 00:08:33.159
And so if you think about
it, what part of the earth

00:08:33.159 --> 00:08:34.620
is being lit by sunlight?

00:08:34.620 --> 00:08:36.490
Or what part of the
earth is in daylight,

00:08:36.490 --> 00:08:38.570
the way I've drawn
it right over here?

00:08:38.570 --> 00:08:41.600
Well, the part that
is facing the sun.

00:08:41.600 --> 00:08:46.190
So all of this right over here
is going to be in daylight.

00:08:46.190 --> 00:08:49.140
As we rotate whatever part
of the surface of the earth

00:08:49.140 --> 00:08:51.540
enters into this yellow
part right over here

00:08:51.540 --> 00:08:53.427
will be in daylight.

00:08:53.427 --> 00:08:55.010
But let's think about
what's happening

00:08:55.010 --> 00:08:55.920
at different parts of the earth.

00:08:55.920 --> 00:08:58.253
So let me draw the equator,
which separates our Northern

00:08:58.253 --> 00:08:59.880
and Southern Hemispheres.

00:08:59.880 --> 00:09:02.965
So this is the equator.

00:09:05.501 --> 00:09:07.500
And then let me go into
the Northern Hemisphere.

00:09:07.500 --> 00:09:10.870
And I want to show you why when
the North Pole is pointed away

00:09:10.870 --> 00:09:13.722
from the sun why
this is our winter.

00:09:13.722 --> 00:09:15.430
So when we're pointed
away from the sun--

00:09:15.430 --> 00:09:17.480
Well, if we go to the
Arctic Circle-- so let

00:09:17.480 --> 00:09:20.430
me go right over here.

00:09:20.430 --> 00:09:23.640
Let me go to some point
in the Arctic Circle.

00:09:23.640 --> 00:09:27.800
As it goes, as the earth
rotates every 24 hours,

00:09:27.800 --> 00:09:31.030
this point on the globe
will just rotate around just

00:09:31.030 --> 00:09:32.000
like that.

00:09:32.000 --> 00:09:34.832
It will just keep rotating
around just like that.

00:09:34.832 --> 00:09:36.790
And so my question is,
that point in the Arctic

00:09:36.790 --> 00:09:39.460
Circle, as it rotates
will it ever see sunlight?

00:09:39.460 --> 00:09:41.300
Well, no, it will
never see sunlight.

00:09:41.300 --> 00:09:45.110
Because the North Pole is
tilted away from the sun.

00:09:45.110 --> 00:09:49.590
So what I'm drawing, what
I'm shading here in purple,

00:09:49.590 --> 00:09:52.070
that part of the earth, when
it's completely tilted away,

00:09:52.070 --> 00:09:54.086
will never see sunlight.

00:09:54.086 --> 00:09:55.460
Or at least it
won't see sunlight

00:09:55.460 --> 00:09:58.430
while it's tilted away,
while it's in this position,

00:09:58.430 --> 00:10:01.000
or in this position
in the orbit.

00:10:01.000 --> 00:10:03.080
I won't say never, because
once it becomes summer

00:10:03.080 --> 00:10:04.246
they will be able to see it.

00:10:04.246 --> 00:10:10.580
So no sunlight, no day, I guess
you could say, no daylight.

00:10:10.580 --> 00:10:14.210
If you go to slightly
more southern latitudes,

00:10:14.210 --> 00:10:15.860
so let's say you go over here.

00:10:15.860 --> 00:10:18.570
So maybe that's the latitude of
something like, I don't know,

00:10:18.570 --> 00:10:21.360
New York or San Francisco
or something like that.

00:10:21.360 --> 00:10:23.890
Let's think about what it
would see as the earth rotates

00:10:23.890 --> 00:10:25.510
every 24 hours.

00:10:25.510 --> 00:10:28.020
So this would be daylight,
daylight, daylight, daylight,

00:10:28.020 --> 00:10:30.500
then nighttime, nighttime,
nighttime, nighttime,

00:10:30.500 --> 00:10:31.077
nighttime.

00:10:31.077 --> 00:10:33.410
This is now going behind the
globe nighttime, nighttime,

00:10:33.410 --> 00:10:36.050
nighttime, nighttime, nighttime,
daylight, daylight, daylight,

00:10:36.050 --> 00:10:37.140
daylight.

00:10:37.140 --> 00:10:38.796
So if you just compare this.

00:10:38.796 --> 00:10:40.295
So let me do the
daylight in orange.

00:10:43.090 --> 00:10:48.170
And then nighttime I will do
in this bluish purplish color.

00:10:48.170 --> 00:10:50.260
So night time over here.

00:10:50.260 --> 00:10:52.560
So if you go to really
northern latitudes,

00:10:52.560 --> 00:10:55.656
like the Arctic Circle,
they don't get any daylight

00:10:55.656 --> 00:10:57.280
when we are tilted
away from the earth.

00:10:57.280 --> 00:11:01.270
And if we go to slightly
still northern latitudes,

00:11:01.270 --> 00:11:04.080
but not as north as the Arctic
Circle, it does get daylight.

00:11:04.080 --> 00:11:05.510
But it gets a lot less daylight.

00:11:05.510 --> 00:11:08.000
It spends a lot less
time in the daylight

00:11:08.000 --> 00:11:09.530
than in the night time.

00:11:09.530 --> 00:11:14.070
So notice if you say that
this circumference represents

00:11:14.070 --> 00:11:17.950
the positions over 24 hours,
it spends much less time

00:11:17.950 --> 00:11:24.740
in the daylight than it
does in the nighttime.

00:11:24.740 --> 00:11:28.560
So because, while the Northern
Hemisphere is tilted away

00:11:28.560 --> 00:11:32.670
from the earth, the latitudes
in the northern hemisphere

00:11:32.670 --> 00:11:33.875
are getting less daylight.

00:11:37.870 --> 00:11:40.660
They are also getting
less energy from the sun.

00:11:40.660 --> 00:11:42.890
And so that's what
leads to winter,

00:11:42.890 --> 00:11:45.120
or just being generally colder.

00:11:45.120 --> 00:11:46.760
And to see what
happens in the summer

00:11:46.760 --> 00:11:48.380
let's just go the other side.

00:11:48.380 --> 00:11:51.040
So now we're going to the
other side of our orbit

00:11:51.040 --> 00:11:51.940
around the sun.

00:11:51.940 --> 00:11:53.680
This is going to be
six months later.

00:11:53.680 --> 00:11:55.950
And notice the actual
direction, relative to the rest

00:11:55.950 --> 00:11:57.910
of the universe,
has not changed.

00:11:57.910 --> 00:11:59.890
We're still pointed in
that same direction.

00:11:59.890 --> 00:12:04.660
We still have a 23.4 degree
tilt relative to, I guess,

00:12:04.660 --> 00:12:06.220
being straight up and down.

00:12:06.220 --> 00:12:08.590
But now once we're over
here the light from the sun

00:12:08.590 --> 00:12:10.180
is going to be coming
from the right.

00:12:15.670 --> 00:12:16.890
Just like that.

00:12:16.890 --> 00:12:18.880
And now, if on this
diagram at least,

00:12:18.880 --> 00:12:21.020
this is the side
of the earth that

00:12:21.020 --> 00:12:24.560
is going to be
getting the sunlight.

00:12:24.560 --> 00:12:27.210
And let me draw
the equator again,

00:12:27.210 --> 00:12:29.984
or my best attempt
to draw the equator.

00:12:29.984 --> 00:12:31.650
I'll draw the equator
in that same color

00:12:31.650 --> 00:12:34.590
actually, in that green color.

00:12:34.590 --> 00:12:38.750
So this separates the Northern
and the Southern Hemisphere.

00:12:38.750 --> 00:12:41.170
And now let's think
about the Arctic Circle.

00:12:41.170 --> 00:12:43.640
So let's say I'm sitting
here in the Arctic Circle.

00:12:43.640 --> 00:12:46.300
As the day goes on,
as 24 hours go around,

00:12:46.300 --> 00:12:47.760
I'll keep rotating around here.

00:12:47.760 --> 00:12:51.850
But notice the whole time
I am inside of the sun.

00:12:51.850 --> 00:12:55.470
I'm getting no nighttime.

00:12:55.470 --> 00:12:58.400
There is no night
in the Arctic Circle

00:12:58.400 --> 00:13:00.820
while we are tilted
towards the sun.

00:13:00.820 --> 00:13:03.780
And if we still do that
fairly northern latitude,

00:13:03.780 --> 00:13:05.280
but not as far as
the Arctic Circle,

00:13:05.280 --> 00:13:09.520
maybe in San Francisco or New
York, or something like that .

00:13:09.520 --> 00:13:12.240
If we go to that latitude,
notice how much time

00:13:12.240 --> 00:13:13.580
we spend in the sun.

00:13:13.580 --> 00:13:15.550
So maybe we just enter.

00:13:15.550 --> 00:13:18.260
So this is right at sunrise.

00:13:18.260 --> 00:13:21.259
And then as the
day goes on we're

00:13:21.259 --> 00:13:23.842
in sunlight, sunlight, sunlight,
sunlight, sunlight, sunlight,

00:13:23.842 --> 00:13:25.200
sunlight, sunlight.

00:13:25.200 --> 00:13:27.100
Then we hit sunset.

00:13:27.100 --> 00:13:29.480
Then we hit
nighttime, nighttime,

00:13:29.480 --> 00:13:32.821
then we hit nighttime, and
then we get sunrise again.

00:13:32.821 --> 00:13:34.570
And so when you look
at the amount of time

00:13:34.570 --> 00:13:36.480
that something in the
Northern Hemisphere

00:13:36.480 --> 00:13:38.470
spends in the daylight
versus sunlight,

00:13:38.470 --> 00:13:41.870
you'll see it spends a lot
more time in the daylight when

00:13:41.870 --> 00:13:45.100
the Northern Hemisphere
is tilted towards the sun.

00:13:45.100 --> 00:13:53.930
So this is more day, less night.

00:13:53.930 --> 00:13:56.190
So it is getting more
energy from the sun.

00:13:56.190 --> 00:13:57.690
So when it is tilted
towards the sun

00:13:57.690 --> 00:14:00.030
it is getting more
energy from the sun.

00:14:00.030 --> 00:14:02.240
So things will
generally be warmer.

00:14:02.240 --> 00:14:08.560
And so you are now
talking about summer

00:14:08.560 --> 00:14:09.960
in the Northern Hemisphere.

00:14:09.960 --> 00:14:11.834
And the arguments for
the Southern Hemisphere

00:14:11.834 --> 00:14:12.440
are identical.

00:14:12.440 --> 00:14:14.064
You could even play
it right over here.

00:14:14.064 --> 00:14:16.690
When the Northern Hemisphere
is tilted away from the sun,

00:14:16.690 --> 00:14:19.674
then the Southern Hemisphere
is tilted towards the sun.

00:14:19.674 --> 00:14:21.090
And so for example,
the South Pole

00:14:21.090 --> 00:14:23.230
will have all daylight
and no nighttime.

00:14:23.230 --> 00:14:26.770
And southern latitudes will have
more daylight than nighttime.

00:14:26.770 --> 00:14:28.870
And so the south
will have summer.

00:14:28.870 --> 00:14:31.405
So this is summer in the south,
in the Southern Hemisphere.

00:14:31.405 --> 00:14:32.940
And it's winter in the north.

00:14:32.940 --> 00:14:34.690
And then down here the
Southern Hemisphere

00:14:34.690 --> 00:14:36.670
is pointed away from the sun.

00:14:36.670 --> 00:14:40.707
So this is winter in
the Southern Hemisphere.

00:14:40.707 --> 00:14:43.040
And you might be saying, hey
Sal, what about you haven't

00:14:43.040 --> 00:14:45.360
talked a lot about
spring and fall.

00:14:45.360 --> 00:14:46.730
Well let's think about it.

00:14:46.730 --> 00:14:48.938
Well, if we're talking about
the Northern Hemisphere,

00:14:48.938 --> 00:14:51.140
this over here, we
decided, was winter

00:14:51.140 --> 00:14:52.940
in the Northern Hemisphere.

00:14:52.940 --> 00:14:55.110
And we're going to
rotate around the sun.

00:14:55.110 --> 00:14:58.840
And at some point, we're
going to get over here.

00:14:58.840 --> 00:15:02.490
And then because of
this tilt we aren't

00:15:02.490 --> 00:15:04.130
pointed away or towards the sun.

00:15:04.130 --> 00:15:07.570
We're kind of pointed
I guess sideways

00:15:07.570 --> 00:15:09.330
relative to the
direction of the sun.

00:15:09.330 --> 00:15:13.990
But this doesn't favor one
hemisphere over the other.

00:15:13.990 --> 00:15:16.600
So when we're over here
in-- and this will actually

00:15:16.600 --> 00:15:21.700
be the spring now- when
we're in the spring,

00:15:21.700 --> 00:15:24.850
both hemispheres are getting
the equal amount of daylight

00:15:24.850 --> 00:15:26.870
and sunlight, or
for a given latitude

00:15:26.870 --> 00:15:28.520
above or below the
equator, they're

00:15:28.520 --> 00:15:29.890
getting the same amount.

00:15:29.890 --> 00:15:31.490
And the same thing
is true over here

00:15:31.490 --> 00:15:33.374
when we get to--
so this is spring.

00:15:33.374 --> 00:15:35.290
This is the summer in
the Northern Hemisphere.

00:15:35.290 --> 00:15:39.770
Now this will be the fall
in the Northern Hemisphere.

00:15:39.770 --> 00:15:42.850
And once again, we're
tilted in this direction.

00:15:42.850 --> 00:15:44.660
And so the Northern
Hemisphere isn't

00:15:44.660 --> 00:15:48.646
tilted away or towards the sun.

00:15:48.646 --> 00:15:50.020
And so both
hemispheres are going

00:15:50.020 --> 00:15:53.770
to get the same amount of
radiation from the sun.

00:15:53.770 --> 00:15:56.710
So you really see the extremes
in the winters and the summers.

00:15:56.710 --> 00:15:58.600
Now one thing I do
want to make clear,

00:15:58.600 --> 00:16:01.659
and I started off with just the
length of day and nighttime.

00:16:01.659 --> 00:16:03.950
Because frankly, that's maybe
a little bit, or at least

00:16:03.950 --> 00:16:05.990
in my brain, a little
bit easier to visualize.

00:16:05.990 --> 00:16:07.750
But that by itself
does not account

00:16:07.750 --> 00:16:10.820
for all of the difference
between summer and winter.

00:16:10.820 --> 00:16:14.180
Another cause, and actually this
is probably the biggest cause,

00:16:14.180 --> 00:16:16.320
is if you think about
the total amount of sun.

00:16:16.320 --> 00:16:19.657
So let's talk about the
Northern Hemisphere winter.

00:16:19.657 --> 00:16:21.740
And let's say there's a
certain amount of sunlight

00:16:21.740 --> 00:16:22.985
that is reaching the earth.

00:16:26.350 --> 00:16:28.020
So this is the total
amount of sunlight

00:16:28.020 --> 00:16:32.310
that's reaching the earth
at any point in time.

00:16:32.310 --> 00:16:35.230
You see that much more of
that is hitting the Southern

00:16:35.230 --> 00:16:37.930
Hemisphere than the
Northern Hemisphere here.

00:16:37.930 --> 00:16:40.330
All of these, if
you imagine it, all

00:16:40.330 --> 00:16:42.700
of these rays
right over here are

00:16:42.700 --> 00:16:45.307
hitting the Southern Hemisphere.

00:16:45.307 --> 00:16:47.390
So a majority of the rays
are hitting the Southern

00:16:47.390 --> 00:16:48.100
Hemisphere.

00:16:48.100 --> 00:16:50.760
And much fewer are hitting
the Northern Hemisphere.

00:16:50.760 --> 00:16:53.270
So actually a smaller amount
of the radiation period,

00:16:53.270 --> 00:16:55.450
at even a given period
in time, not even talking

00:16:55.450 --> 00:16:58.750
about the amount of time
you are facing the sun.

00:16:58.750 --> 00:17:01.330
But at any given moment
in time more energy

00:17:01.330 --> 00:17:04.500
is hitting the Southern
Hemisphere than the Northern.

00:17:04.500 --> 00:17:08.970
And the opposite is true
when the tilt is then

00:17:08.970 --> 00:17:09.810
towards the sun.

00:17:09.810 --> 00:17:16.900
And now a disproportionate
amount of the sun's energy

00:17:16.900 --> 00:17:19.460
is hitting the
Northern Hemisphere.

00:17:19.460 --> 00:17:21.460
So if you draw a
bunch of, if you just

00:17:21.460 --> 00:17:24.300
think that this is all of
the energy from the sun, most

00:17:24.300 --> 00:17:25.859
of it, all of
these rays up here,

00:17:25.859 --> 00:17:27.369
are hitting the
Northern Hemisphere.

00:17:27.369 --> 00:17:31.060
And only these down here
are hitting the Southern

00:17:31.060 --> 00:17:31.980
Hemisphere.

00:17:31.980 --> 00:17:34.390
And on top of that, what
makes it even more extreme

00:17:34.390 --> 00:17:37.970
is that the actual angle, and of
course, this is to some degree

00:17:37.970 --> 00:17:40.940
is due to the fact that
where the angle of the sun

00:17:40.940 --> 00:17:44.420
relative to the horizon,
or where you are on Earth.

00:17:44.420 --> 00:17:46.530
But even more than
that if you are on,

00:17:46.530 --> 00:17:50.860
let's say that this
is the land, and we're

00:17:50.860 --> 00:17:53.600
talking about the winter
in the Northern Hemisphere.

00:17:53.600 --> 00:17:55.800
So let's say you're
talking about,

00:17:55.800 --> 00:18:00.960
let's say we're up over here
at this northern latitude.

00:18:00.960 --> 00:18:04.980
And we're just looking
at the sun here.

00:18:04.980 --> 00:18:06.460
And over here,
you could see even

00:18:06.460 --> 00:18:09.570
when we are closest to the
sun the sun is not directly

00:18:09.570 --> 00:18:10.569
overhead.

00:18:10.569 --> 00:18:12.110
When we're closest
to the sun the sun

00:18:12.110 --> 00:18:14.450
still is pretty
low on the horizon.

00:18:14.450 --> 00:18:16.230
So it may be right
over here when

00:18:16.230 --> 00:18:19.150
we're closest to the sun
in the winter the sun might

00:18:19.150 --> 00:18:21.740
be right over here.

00:18:21.740 --> 00:18:26.340
But if you look at that
same latitude in the summer

00:18:26.340 --> 00:18:30.440
when it is closest
to the sun, the sun

00:18:30.440 --> 00:18:33.370
is more close to being
directly overhead.

00:18:33.370 --> 00:18:35.300
It still won't be
directly overhead.

00:18:35.300 --> 00:18:39.020
Because we are still at a
relatively northern latitude.

00:18:39.020 --> 00:18:45.120
But the sun is going to
be much higher in the sky.

00:18:45.120 --> 00:18:46.799
And these are all
related to each other.

00:18:46.799 --> 00:18:48.340
It's kind of connected
with this idea

00:18:48.340 --> 00:18:52.830
that more energy is hitting
one hemisphere or the other.

00:18:52.830 --> 00:18:55.180
But also, when you
have a, I guess

00:18:55.180 --> 00:18:58.080
you could say, a steeper
angle from the rays

00:18:58.080 --> 00:19:00.030
of the sun with the
earth, it's actually

00:19:00.030 --> 00:19:02.960
going to be dissipated
less by the atmosphere.

00:19:02.960 --> 00:19:05.600
And let me just make
it clear how this is.

00:19:05.600 --> 00:19:10.750
So in the summer-- so let's
say that that's the land.

00:19:10.750 --> 00:19:16.070
And let's say that-- let me
draw the atmosphere in white--

00:19:16.070 --> 00:19:18.929
so all of this area right over
here, this is the atmosphere.

00:19:18.929 --> 00:19:21.470
And obviously there's not a hard
boundary for the atmosphere.

00:19:21.470 --> 00:19:24.760
But let's just say this is the
densest part of the atmosphere.

00:19:24.760 --> 00:19:32.480
In the summer, when the
sun is higher in the sky,

00:19:32.480 --> 00:19:35.730
the rays from the sun are
dissipated by less atmosphere.

00:19:35.730 --> 00:19:38.300
So they have to get through
this much atmosphere.

00:19:38.300 --> 00:19:39.300
And they're bounced off.

00:19:39.300 --> 00:19:41.970
And they heat some
of that atmosphere.

00:19:41.970 --> 00:19:44.530
And they're absorbed before
they get to the ground.

00:19:44.530 --> 00:19:49.340
In the winter when the
sun is lower in the sky,

00:19:49.340 --> 00:19:51.994
so maybe the sun is out here.

00:19:51.994 --> 00:19:53.160
Let me draw it a little bit.

00:19:53.160 --> 00:19:57.830
So when the sun is lower in
the sky relative to this point,

00:19:57.830 --> 00:19:59.490
you see that the
rays of sunlight

00:19:59.490 --> 00:20:04.130
have to travel through
a lot more atmosphere.

00:20:04.130 --> 00:20:07.020
So they get dissipated
much more before they

00:20:07.020 --> 00:20:09.460
get to this point on the planet.

00:20:09.460 --> 00:20:11.690
So all in all it
is the tilt that

00:20:11.690 --> 00:20:13.930
is causing the
changes in the season.

00:20:13.930 --> 00:20:16.190
But it's causing it
for multiple reasons.

00:20:16.190 --> 00:20:18.660
One is when you're
tilted, we'll say

00:20:18.660 --> 00:20:20.590
when you're tilted
towards the sun,

00:20:20.590 --> 00:20:23.840
you're getting more
absolute hours of daylight.

00:20:23.840 --> 00:20:26.340
Not only are you getting more
absolute hours of daylight,

00:20:26.340 --> 00:20:30.686
but at any given moment, most
or more of the sun's total rays

00:20:30.686 --> 00:20:32.810
that are hitting the earth
are hitting the Northern

00:20:32.810 --> 00:20:35.050
Hemisphere as opposed to
the Southern Hemisphere.

00:20:35.050 --> 00:20:40.350
And the stuff that's hitting
the places that have summer,

00:20:40.350 --> 00:20:42.740
it has to go through
less atmosphere.

00:20:42.740 --> 00:20:45.597
So it gets dissipated less.


Seasons aren't dictated by closeness to sun

WEBVTT
Kind: captions
Language: en

00:00:00.610 --> 00:00:04.500
If you were to ask some
people why we have seasons,

00:00:04.500 --> 00:00:07.740
they might say that maybe
it's due to how far we

00:00:07.740 --> 00:00:11.110
are from the sun at
different points in the year,

00:00:11.110 --> 00:00:13.362
different points
in Earth's orbit.

00:00:13.362 --> 00:00:14.820
And what I want to
do in this video

00:00:14.820 --> 00:00:17.440
is show you why
that isn't the case.

00:00:17.440 --> 00:00:19.810
So the line of reasoning
would go something like this.

00:00:19.810 --> 00:00:22.740
This is the sun at the center
of our solar system and roughly

00:00:22.740 --> 00:00:24.710
at the center of Earth's orbit.

00:00:24.710 --> 00:00:27.771
And let me draw Earth's
orbit over here.

00:00:27.771 --> 00:00:29.520
And so the line of
reasoning is that there

00:00:29.520 --> 00:00:30.900
are certain points
in Earth's orbit

00:00:30.900 --> 00:00:32.900
where we are closer to
the sun and certain-- let

00:00:32.900 --> 00:00:34.470
me draw a better job than that.

00:00:34.470 --> 00:00:36.886
So let's say this the point
where we're closer to the sun.

00:00:36.886 --> 00:00:37.890
We get a little further.

00:00:37.890 --> 00:00:38.973
Then we get a lot further.

00:00:38.973 --> 00:00:40.480
And then we get a
little bit closer.

00:00:40.480 --> 00:00:42.672
And then a little bit closer.

00:00:42.672 --> 00:00:44.130
And then this is
the closest point.

00:00:44.130 --> 00:00:48.090
So maybe Earth's orbit
looks something like this.

00:00:50.379 --> 00:00:51.920
So the argument
would go, look, there

00:00:51.920 --> 00:00:53.670
are points in
Earth's orbit where

00:00:53.670 --> 00:00:55.180
we're closer to
the sun and points

00:00:55.180 --> 00:00:56.680
where we are further
from the sun.

00:00:56.680 --> 00:00:59.200
And actually that part
of the argument is true.

00:00:59.200 --> 00:01:01.720
Earth's orbit is not
a perfect circle.

00:01:01.720 --> 00:01:03.360
And there are points
in Earth's orbit

00:01:03.360 --> 00:01:06.100
where we are closer or
further away from the sun.

00:01:06.100 --> 00:01:08.300
And actually when we
are closest to the sun,

00:01:08.300 --> 00:01:11.870
so if Earth is right over
here, there's a word for that.

00:01:11.870 --> 00:01:12.860
It's called perihelion.

00:01:12.860 --> 00:01:16.110
It just means the closest
point in orbit, perihelion.

00:01:19.720 --> 00:01:22.070
Closest point in
orbit to the sun.

00:01:22.070 --> 00:01:25.040
And there is a furthest
point from the sun,

00:01:25.040 --> 00:01:26.320
and this is called aphelion.

00:01:26.320 --> 00:01:27.250
Or aphelion.

00:01:27.250 --> 00:01:30.260
I've sometimes seen it called
aphelion, pronounced ap-helion.

00:01:33.740 --> 00:01:36.210
So it is true that
Earth's orbit is not

00:01:36.210 --> 00:01:37.650
a perfect circle around the sun.

00:01:37.650 --> 00:01:40.330
Although it's pretty close,
but it's not a perfect circle.

00:01:40.330 --> 00:01:44.420
It has a slightly
elliptical shape.

00:01:44.420 --> 00:01:46.210
And because of that,
there are times

00:01:46.210 --> 00:01:48.169
in the year where we
are closest to the sun,

00:01:48.169 --> 00:01:49.710
and there are times
of the year where

00:01:49.710 --> 00:01:50.980
we are furthest to the sun.

00:01:50.980 --> 00:01:52.980
And the difference is about 3%.

00:01:52.980 --> 00:01:54.880
So it's not a huge
difference in distance.

00:01:54.880 --> 00:01:56.760
I've really exaggerated
the difference

00:01:56.760 --> 00:01:59.260
in this diagram right over here.

00:01:59.260 --> 00:02:01.820
But based on this reasoning,
people would say--

00:02:01.820 --> 00:02:03.640
and this is the flawed
part-- that when

00:02:03.640 --> 00:02:05.900
we're closer to the sun,
this must be the summer.

00:02:08.830 --> 00:02:11.430
And when we are furthest
away from the sun,

00:02:11.430 --> 00:02:12.580
this must be the winter.

00:02:15.850 --> 00:02:20.080
And the most obvious point
of evidence why this is not

00:02:20.080 --> 00:02:23.037
the case is that
when it is summer

00:02:23.037 --> 00:02:24.870
at one point on the
planet, it is not summer

00:02:24.870 --> 00:02:27.530
throughout the
planet at that time.

00:02:27.530 --> 00:02:29.200
So let me draw our planet.

00:02:32.490 --> 00:02:39.290
In particular, when it is summer
in the Northern Hemisphere--

00:02:39.290 --> 00:02:43.740
actually, let me do summer
in a more warm color.

00:02:43.740 --> 00:02:46.680
When it is summer in the
Northern Hemisphere-- so here,

00:02:46.680 --> 00:02:51.155
it is summer-- it is winter
in the Southern Hemisphere.

00:02:59.480 --> 00:03:02.020
And when it is summer in
the Southern Hemisphere,

00:03:02.020 --> 00:03:03.720
it is winter in the
Northern Hemisphere.

00:03:03.720 --> 00:03:06.740
So the entire planet does
not experience the seasons

00:03:06.740 --> 00:03:08.360
at the same time.

00:03:08.360 --> 00:03:10.610
So that's probably, I
guess you could say,

00:03:10.610 --> 00:03:13.850
the biggest point of data that
we observe on our planet why

00:03:13.850 --> 00:03:17.920
this by itself cannot explain
the change in seasons.

00:03:17.920 --> 00:03:21.670
And in particular,
it really goes

00:03:21.670 --> 00:03:24.200
against what we experience
in the Northern Hemisphere,

00:03:24.200 --> 00:03:28.620
because our perihelion right
now is occurring in January.

00:03:28.620 --> 00:03:32.770
It is occurring during the
winter, the Northern Hemisphere

00:03:32.770 --> 00:03:33.730
winter.

00:03:33.730 --> 00:03:41.510
Perihelion in right now is
during the Northern Hemisphere

00:03:41.510 --> 00:03:42.360
winter.

00:03:42.360 --> 00:03:44.350
And when we are furthest
away from the sun,

00:03:44.350 --> 00:03:46.370
this is actually the
Northern Hemisphere summer.

00:03:52.170 --> 00:03:54.677
So although it might seem like
a fairly intuitive idea, hey,

00:03:54.677 --> 00:03:57.260
if we're closer to the sun, the
whole planet's getting warmer,

00:03:57.260 --> 00:03:58.320
maybe that's summer.

00:03:58.320 --> 00:04:00.220
When we're further
away, the whole planet's

00:04:00.220 --> 00:04:02.630
getting a little less
energy, that's winter.

00:04:02.630 --> 00:04:05.120
The evidence we see on Earth
goes directly against that.

00:04:05.120 --> 00:04:07.322
In particular, we don't
have the same seasons

00:04:07.322 --> 00:04:09.155
in both the Northern
and Southern Hemisphere

00:04:09.155 --> 00:04:10.580
at the same time.

00:04:10.580 --> 00:04:12.620
And in particular, in
the Northern Hemisphere,

00:04:12.620 --> 00:04:15.530
when we're closest to the
sun, it's actually in January.

00:04:15.530 --> 00:04:17.730
It's actually in the
middle of winter.

00:04:17.730 --> 00:04:20.329
So I'll leave you
there in this video,

00:04:20.329 --> 00:04:23.600
I've left you just saying, OK,
so the closeness to the sun

00:04:23.600 --> 00:04:26.920
does not dictate what
season we are in.

00:04:26.920 --> 00:04:28.750
And so you're saying,
what is the reason?

00:04:28.750 --> 00:04:31.850
And what we'll see
in the next video,

00:04:31.850 --> 00:04:34.750
the reason is the
tilt of the axis

00:04:34.750 --> 00:04:38.800
of the earth, the rotational
axis of the earth.