- [Instructor] In this video, we're gonna think about
why we have seasons on Earth,
like summer and fall and winter and spring.
Now, one theory that some folks might have
is maybe it's due to the distance
between the Earth and the sun.
We know that the Earth orbits the sun,
the sun is where the great majority of the heat
and the energy on the surface of the Earth comes from.
And maybe it's the case
that there's certain times of year
when we are further from the sun,
and there's other times of year
when we are closer to the sun.
Well, this doesn't actually hold up to why we have seasons
because first of all, when the Northern Hemisphere
the top half of Earth has winter,
the bottom hemisphere has summer and vice versa.
So it can't just be due to the distance of the whole planet,
it also turns out
that when Earth is furthest from the sun is in July,
which is in the middle of summer
in the Northern Hemisphere,
and when we are closest to the sun
is actually in January
which we know tends to be our colder season
in the Northern Hemisphere.
So distance to the sun does not hold up
as to why we have seasons.
The real reason why we have seasons
is because of Earth's axial tilt,
I guess you could say that or it's rotational tilt.
Now this picture shows that tilt
but before we go into it, I'd like to remind folks
that this is nowhere near drawn at scale.
The actual sun has a diameter over 100 times out of Earth,
a million Earths can fit in the sun
and the actual distance between the Earth and the sun
is over 100 times the diameter between the sun and Earth.
But going back to tilt
and you could see that here in this picture,
Earth's north pole does not point straight up
from the plane of Earth's orbit around the sun.
What do I mean by the plane of Earth's orbit around the sun?
This red circle that you see,
or this ellipse that you see,
if you imagine that being on a surface
of a table or a plane,
that would be our orbital plane.
And we can see that the north pole
does not go straight up from that
and the south pole does not go straight down,
that actually we're looking
at an angle of about 23.5 degrees.
And that's the reason why we have the seasons.
To understand why that is the case,
let's imagine Earth when the Northern Hemisphere
is most pointed towards the sun,
which happens in late June.
And so let me draw the equator
to help us visualize this a little bit
and let's compare that
to when the Northern Hemisphere
is most pointed away from the sun,
which happens in late December.
And so I will draw the equator again
to help us visualize this.
And let's pick a similar point in the Northern Hemisphere.
So let's pick a point that's a little bit above the equator.
So let's say that point
and a comparable point in this scenario
is going to be right over here.
It's about that same distance above the equator.
Notice, in late June in the Northern Hemisphere
the sun is almost directly above
this white point that we're seeing here.
While in this scenario, the sun is at an angle.
The surface of the Earth is more like this,
so the sun's rays are coming at an angle.
And if you think about it,
think about the scenario,
the difference between when the sun
is directly bearing down on something,
versus when it is coming at an angle.
Let's say this is a side view of two surfaces.
And the surface on the right has twice the surface area.
You can see the side view has twice the length,
so the surface area if you were to see it in 3D
would be twice the surface area
of what we have on the left here.
But if you have the same amount of sun
coming from the same direction,
so here, let me just draw three sun rays here,
this is just indicative
and let me draw three sun rays here.
Notice you have this same amount of energy
but here you're hitting twice the surface area.
So, the amount of energy per unit surface area
is gonna be half as much in this scenario
where the sun is coming at an angle
versus this scenario where the sunlight
is coming more directly on top of that point.
And wherever you go in the Northern Hemisphere
the angle is less direct in the winter
than it is in the summer.
Now there's also some effects
on the amount of daylight you get.
For example, in the summer,
when the Northern Hemisphere
is most tilted towards the Earth,
in the north pole, you're gonna have constant daylight
and the south pole you're gonna have constant nighttime.
And then the opposite happens
when the Northern Hemisphere is pointed away.
And then when we think about spring and autumn
in either hemisphere,
you can see that the angle of Earth's rotation
does not change from that 23.5 degrees,
but in spring and autumn
the Northern Hemisphere is not pointed
to or away from the sun,
it's kind of just pointed to the side.
So in these two points,
comparable points on the Northern or Southern hemisphere
are seeing similar angles of the actual sunlight.