How earth’s tilt causes seasons | Cosmology & Astronomy | Khan Academy


In the last video, we talk
about how seasons on Earth are not caused by
how close Earth is to the sun in its orbit. And we also hint at the
fact that it’s actually caused by the tilt of the earth. And so in this video,
I want to show you how the tilt of the earth
causes the seasons to happen. So let’s draw– so
I’m going to try to draw as many diagrams
as possible here. Because at least
for my brain they help me visualize what’s
actually going on. So we could imagine
a top view first. So let’s have a top view. That is the sun
right over there. And let me draw
the earth’s orbit. So Earth’s orbit maybe
looks something like that. Let me draw it almost,
it is almost circular. So I’ll draw it as
something that’s pretty close to a
circle right over here. And I’m going to draw Earth at
different points in its orbit. And I’m going to try
to depict the tilt of its rotational axis. And obviously, this is not drawn
anywhere near close to scale. Earth is much further
away from the sun, and much, much smaller
than the sun as well. So I’ll draw the
earth at that point. And at this point,
the earth will be tilted away from the sun. So Earth’s tilt does
not change if you think about the
direction, or at least over the course of
a year, if we think about relatively
small periods of time. It does not change
relative to the direction that it’s pointing
at in the universe. And we’ll talk about
that in a second. So let’s say right over here we
are pointed away from the sun. So we’re up and
out of this page. So if I wanted to put some
perspective on an arrow it would be up and– actually
it would be more like up and out of this page. So that’s the direction. If you were to come straight
out of the North Pole. And if you were to go
straight out of the South Pole you’d go below that
circle right over there. And if I wanted to
draw the same position, but if we’re looking
sideways along the plane, the orbital plane, or the
plane of Earth’s orbit. So if we’re looking at
it from that direction. So let me do it this way. If we’re looking at
directly sideways, this is the sun right
over here, and this is Earth at that position. This is Earth right over there. If I were to draw an arrow
pointing straight out of the North Pole it would
look something like this. So this arrow and
this arrow they are both popping straight
out of the North Pole. And so when we talk about
the tilt of the earth, we’re talking about the
tilt of its orbital axis, kind of this pole that could
go straight between the South Pole and the North Pole. The angle between that and
a pole that would actually be at a 90-degree
angle, or perpendicular, to the plane of its orbit. And so compared to if it was
just straight up and down, relative to the
plane of the orbit. So this right here is the
angle of Earth’s tilt. Let me draw that a
little bit bigger just so it becomes a
little bit clearer. So if this is the
plane of the orbit, we’re looking sideways along
the plane of the orbit. And this is Earth
right over here. My best attempt
to draw a circle. That is Earth. Earth does not rotate. Its axis of rotation is not
perpendicular to the plane of the orbit. So this is how
Earth would orbit. This is how Earth
would rotate if it was. Earth rotates, Earth’s
rotational axis is at an angle to
that vertical relative to the plane of its orbit,
I guess you could say it. It rotates at an
angle like this. So this would be the North Pole. That is the South Pole. And so it rotates like this. And that angle relative to
being vertical with respect to the orbital plane, this
angle right here for Earth right now is 23.4 degrees. And if we’re talking about
relatively short periods of time, like our
lifespans, that is constant. But it is actually changing
over long periods of time. That is changing between–
and these are rough numbers– it is changing between 22.1
degrees and 24.5 degrees, if my sources are correct. But that gives a rough estimate
of what it’s changing between. But I want to make
it clear, this is not happening overnight. The period to go from roughly
a 22-degree angle to a 24 and 1/2-degree angle and back
to a 22-degree angle is 41,000 years. And this long-term
change in the tilt, this might play into some of the
long-term climactic change. Maybe it might
contribute, on some level, to some of the ice ages that
have formed over Earth’s past. But for the sake of thinking
about our annual seasons you don’t have to
worry too much, or you don’t have to
worry at all really about this variation. You really just have to
know that it is tilted. And right now it is tilted
at an angle of 23.4 degrees. Now you might say OK, I
understand what the tilt is. But how does that
change the seasons in either the Northern or
that Southern Hemisphere? And to do that, I’m going
to imagine the earth when the Northern Hemisphere is
most tilted away from the sun, and when it is most
tilted towards the sun. So remember this tilt, the
direction this arrow points into relative to the
rest of the universe, if we assume that
this tilt is at 23.4%, it’s not changing
throughout the year. But depending on where
it is in the orbit it’s either going to be
tilting away from the sun, as it is in this
example right over here. Or it will be tilting
towards the sun. I’ll do the towards the
sun in this magenta color, or it would be tilting
towards the sun. So six months later when
the earth is over here, it’s going to, relative to
the rest of the universe, it will be tilted in that same
direction, up out of this page and to the right. Just like it was over here. But now that it’s on the
other side of the sun that makes it tilt a little
bit more towards the sun. If I were to draw
it right over here, it is now tilted
towards the sun. And what I want to think
about is how much sunlight will different parts
of the planet receive. And I’ll focus on the
Northern Hemisphere. But you can make
a similar argument for the Southern Hemisphere. I want to think about how much
sunlight they receive when it’s tilted away or tilted
towards the sun. And so let’s think about
those two situations. So first of all, let’s
think about this situation here where we are tilted
away from the sun. So let me zoom in a little bit. So this is the situation, where
we’re tilted away from the sun. So if this is the vertical,
so let me draw it. I could actually just
use this diagram. But let me make it. So we’re tilted away
from the sun like this. I’m going to do this
in a different color. So if we have an arrow coming
straight out of the North Pole it would look like this. And we are rotating
around like that. So we’re out of the page
on the left-hand side, and then into the page
on the right-hand side. And so we’re rotating
towards the east, constantly. So this arrow is in the
direction of the east. So when we’re at this point
in Earth’s orbit, and actually let me copy and paste this. And I’m going to use
the same exact diagram for the different seasons. So let me copy. And then let me paste
this exact diagram. I’ll do it over here for
two different points. So when we are here
in Earth’s orbit where is the
sunlight coming from? Well, it’s going to be coming
from the left, at least the way I’ve drawn the diagram
right over here. So the sunlight is coming from
the left in this situation. And so if you think about
it, what part of the earth is being lit by sunlight? Or what part of the
earth is in daylight, the way I’ve drawn
it right over here? Well, the part that
is facing the sun. So all of this right over here
is going to be in daylight. As we rotate whatever part
of the surface of the earth enters into this yellow
part right over here will be in daylight. But let’s think about
what’s happening at different parts of the earth. So let me draw the equator,
which separates our Northern and Southern Hemispheres. So this is the equator. And then let me go into
the Northern Hemisphere. And I want to show you why when
the North Pole is pointed away from the sun why
this is our winter. So when we’re pointed
away from the sun– Well, if we go to the
Arctic Circle– so let me go right over here. Let me go to some point
in the Arctic Circle. As it goes, as the earth
rotates every 24 hours, this point on the globe
will just rotate around just like that. It will just keep rotating
around just like that. And so my question is,
that point in the Arctic Circle, as it rotates
will it ever see sunlight? Well, no, it will
never see sunlight. Because the North Pole is
tilted away from the sun. So what I’m drawing, what
I’m shading here in purple, that part of the earth, when
it’s completely tilted away, will never see sunlight. Or at least it
won’t see sunlight while it’s tilted away,
while it’s in this position, or in this position
in the orbit. I won’t say never, because
once it becomes summer they will be able to see it. So no sunlight, no day, I guess
you could say, no daylight. If you go to slightly
more southern latitudes, so let’s say you go over here. So maybe that’s the latitude of
something like, I don’t know, New York or San Francisco
or something like that. Let’s think about what it
would see as the earth rotates every 24 hours. So this would be daylight,
daylight, daylight, daylight, then nighttime, nighttime,
nighttime, nighttime, nighttime. This is now going behind the
globe nighttime, nighttime, nighttime, nighttime, nighttime,
daylight, daylight, daylight, daylight. So if you just compare this. So let me do the
daylight in orange. And then nighttime I will do
in this bluish purplish color. So night time over here. So if you go to really
northern latitudes, like the Arctic Circle,
they don’t get any daylight when we are tilted
away from the earth. And if we go to slightly
still northern latitudes, but not as north as the Arctic
Circle, it does get daylight. But it gets a lot less daylight. It spends a lot less
time in the daylight than in the night time. So notice if you say that
this circumference represents the positions over 24 hours,
it spends much less time in the daylight than it
does in the nighttime. So because, while the Northern
Hemisphere is tilted away from the earth, the latitudes
in the northern hemisphere are getting less daylight. They are also getting
less energy from the sun. And so that’s what
leads to winter, or just being generally colder. And to see what
happens in the summer let’s just go the other side. So now we’re going to the
other side of our orbit around the sun. This is going to be
six months later. And notice the actual
direction, relative to the rest of the universe,
has not changed. We’re still pointed in
that same direction. We still have a 23.4 degree
tilt relative to, I guess, being straight up and down. But now once we’re over
here the light from the sun is going to be coming
from the right. Just like that. And now, if on this
diagram at least, this is the side
of the earth that is going to be
getting the sunlight. And let me draw
the equator again, or my best attempt
to draw the equator. I’ll draw the equator
in that same color actually, in that green color. So this separates the Northern
and the Southern Hemisphere. And now let’s think
about the Arctic Circle. So let’s say I’m sitting
here in the Arctic Circle. As the day goes on,
as 24 hours go around, I’ll keep rotating around here. But notice the whole time
I am inside of the sun. I’m getting no nighttime. There is no night
in the Arctic Circle while we are tilted
towards the sun. And if we still do that
fairly northern latitude, but not as far as
the Arctic Circle, maybe in San Francisco or New
York, or something like that . If we go to that latitude,
notice how much time we spend in the sun. So maybe we just enter. So this is right at sunrise. And then as the
day goes on we’re in sunlight, sunlight, sunlight,
sunlight, sunlight, sunlight, sunlight, sunlight. Then we hit sunset. Then we hit
nighttime, nighttime, then we hit nighttime, and
then we get sunrise again. And so when you look
at the amount of time that something in the
Northern Hemisphere spends in the daylight
versus sunlight, you’ll see it spends a lot
more time in the daylight when the Northern Hemisphere
is tilted towards the sun. So this is more day, less night. So it is getting more
energy from the sun. So when it is tilted
towards the sun it is getting more
energy from the sun. So things will
generally be warmer. And so you are now
talking about summer in the Northern Hemisphere. And the arguments for
the Southern Hemisphere are identical. You could even play
it right over here. When the Northern Hemisphere
is tilted away from the sun, then the Southern Hemisphere
is tilted towards the sun. And so for example,
the South Pole will have all daylight
and no nighttime. And southern latitudes will have
more daylight than nighttime. And so the south
will have summer. So this is summer in the south,
in the Southern Hemisphere. And it’s winter in the north. And then down here the
Southern Hemisphere is pointed away from the sun. So this is winter in
the Southern Hemisphere. And you might be saying, hey
Sal, what about you haven’t talked a lot about
spring and fall. Well let’s think about it. Well, if we’re talking about
the Northern Hemisphere, this over here, we
decided, was winter in the Northern Hemisphere. And we’re going to
rotate around the sun. And at some point, we’re
going to get over here. And then because of
this tilt we aren’t pointed away or towards the sun. We’re kind of pointed
I guess sideways relative to the
direction of the sun. But this doesn’t favor one
hemisphere over the other. So when we’re over here
in– and this will actually be the spring now- when
we’re in the spring, both hemispheres are getting
the equal amount of daylight and sunlight, or
for a given latitude above or below the
equator, they’re getting the same amount. And the same thing
is true over here when we get to–
so this is spring. This is the summer in
the Northern Hemisphere. Now this will be the fall
in the Northern Hemisphere. And once again, we’re
tilted in this direction. And so the Northern
Hemisphere isn’t tilted away or towards the sun. And so both
hemispheres are going to get the same amount of
radiation from the sun. So you really see the extremes
in the winters and the summers. Now one thing I do
want to make clear, and I started off with just the
length of day and nighttime. Because frankly, that’s maybe
a little bit, or at least in my brain, a little
bit easier to visualize. But that by itself
does not account for all of the difference
between summer and winter. Another cause, and actually this
is probably the biggest cause, is if you think about
the total amount of sun. So let’s talk about the
Northern Hemisphere winter. And let’s say there’s a
certain amount of sunlight that is reaching the earth. So this is the total
amount of sunlight that’s reaching the earth
at any point in time. You see that much more of
that is hitting the Southern Hemisphere than the
Northern Hemisphere here. All of these, if
you imagine it, all of these rays
right over here are hitting the Southern Hemisphere. So a majority of the rays
are hitting the Southern Hemisphere. And much fewer are hitting
the Northern Hemisphere. So actually a smaller amount
of the radiation period, at even a given period
in time, not even talking about the amount of time
you are facing the sun. But at any given moment
in time more energy is hitting the Southern
Hemisphere than the Northern. And the opposite is true
when the tilt is then towards the sun. And now a disproportionate
amount of the sun’s energy is hitting the
Northern Hemisphere. So if you draw a
bunch of, if you just think that this is all of
the energy from the sun, most of it, all of
these rays up here, are hitting the
Northern Hemisphere. And only these down here
are hitting the Southern Hemisphere. And on top of that, what
makes it even more extreme is that the actual angle, and of
course, this is to some degree is due to the fact that
where the angle of the sun relative to the horizon,
or where you are on Earth. But even more than
that if you are on, let’s say that this
is the land, and we’re talking about the winter
in the Northern Hemisphere. So let’s say you’re
talking about, let’s say we’re up over here
at this northern latitude. And we’re just looking
at the sun here. And over here,
you could see even when we are closest to the
sun the sun is not directly overhead. When we’re closest
to the sun the sun still is pretty
low on the horizon. So it may be right
over here when we’re closest to the sun
in the winter the sun might be right over here. But if you look at that
same latitude in the summer when it is closest
to the sun, the sun is more close to being
directly overhead. It still won’t be
directly overhead. Because we are still at a
relatively northern latitude. But the sun is going to
be much higher in the sky. And these are all
related to each other. It’s kind of connected
with this idea that more energy is hitting
one hemisphere or the other. But also, when you
have a, I guess you could say, a steeper
angle from the rays of the sun with the
earth, it’s actually going to be dissipated
less by the atmosphere. And let me just make
it clear how this is. So in the summer– so let’s
say that that’s the land. And let’s say that– let me
draw the atmosphere in white– so all of this area right over
here, this is the atmosphere. And obviously there’s not a hard
boundary for the atmosphere. But let’s just say this is the
densest part of the atmosphere. In the summer, when the
sun is higher in the sky, the rays from the sun are
dissipated by less atmosphere. So they have to get through
this much atmosphere. And they’re bounced off. And they heat some
of that atmosphere. And they’re absorbed before
they get to the ground. In the winter when the
sun is lower in the sky, so maybe the sun is out here. Let me draw it a little bit. So when the sun is lower in
the sky relative to this point, you see that the
rays of sunlight have to travel through
a lot more atmosphere. So they get dissipated
much more before they get to this point on the planet. So all in all it
is the tilt that is causing the
changes in the season. But it’s causing it
for multiple reasons. One is when you’re
tilted, we’ll say when you’re tilted
towards the sun, you’re getting more
absolute hours of daylight. Not only are you getting more
absolute hours of daylight, but at any given moment, most
or more of the sun’s total rays that are hitting the earth
are hitting the Northern Hemisphere as opposed to
the Southern Hemisphere. And the stuff that’s hitting
the places that have summer, it has to go through
less atmosphere. So it gets dissipated less.

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