If you were to ask some
people why we have seasons, they might say that maybe
it's due to how far we are from the sun at
different points in the year, different points
in Earth's orbit. And what I want to
do in this video is show you why
that isn't the case. So the line of reasoning
would go something like this. This is the sun at the center
of our solar system and roughly at the center of Earth's orbit. And let me draw Earth's
orbit over here. And so the line of
reasoning is that there are certain points
in Earth's orbit where we are closer to
the sun and certain-- let me draw a better job than that. So let's say this the point
where we're closer to the sun. We get a little further. Then we get a lot further. And then we get a
little bit closer. And then a little bit closer. And then this is
the closest point. So maybe Earth's orbit
looks something like this. So the argument
would go, look, there are points in
Earth's orbit where we're closer to
the sun and points where we are further
from the sun. And actually that part
of the argument is true. Earth's orbit is not
a perfect circle. And there are points
in Earth's orbit where we are closer or
further away from the sun. And actually when we
are closest to the sun, so if Earth is right over
here, there's a word for that. It's called perihelion. It just means the closest
point in orbit, perihelion. Closest point in
orbit to the sun. And there is a furthest
point from the sun, and this is called aphelion. Or aphelion. I've sometimes seen it called
aphelion, pronounced ap-helion. So it is true that
Earth's orbit is not a perfect circle around the sun. Although it's pretty close,
but it's not a perfect circle. It has a slightly
elliptical shape. And because of that,
there are times in the year where we
are closest to the sun, and there are times
of the year where we are furthest to the sun. And the difference is about 3%. So it's not a huge
difference in distance. I've really exaggerated
the difference in this diagram right over here. But based on this reasoning,
people would say-- and this is the flawed
part-- that when we're closer to the sun,
this must be the summer. And when we are furthest
away from the sun, this must be the winter. And the most obvious point
of evidence why this is not the case is that
when it is summer at one point on the
planet, it is not summer throughout the
planet at that time. So let me draw our planet. In particular, when it is summer
in the Northern Hemisphere-- actually, let me do summer
in a more warm color. When it is summer in the
Northern Hemisphere-- so here, it is summer-- it is winter
in the Southern Hemisphere. And when it is summer in
the Southern Hemisphere, it is winter in the
Northern Hemisphere. So the entire planet does
not experience the seasons at the same time. So that's probably, I
guess you could say, the biggest point of data that
we observe on our planet why this by itself cannot explain
the change in seasons. And in particular,
it really goes against what we experience
in the Northern Hemisphere, because our perihelion right
now is occurring in January. It is occurring during the
winter, the Northern Hemisphere winter. Perihelion in right now is
during the Northern Hemisphere winter. And when we are furthest
away from the sun, this is actually the
Northern Hemisphere summer. So although it might seem like
a fairly intuitive idea, hey, if we're closer to the sun, the
whole planet's getting warmer, maybe that's summer. When we're further
away, the whole planet's getting a little less
energy, that's winter. The evidence we see on Earth
goes directly against that. In particular, we don't
have the same seasons in both the Northern
and Southern Hemisphere at the same time. And in particular, in
the Northern Hemisphere, when we're closest to the
sun, it's actually in January. It's actually in the
middle of winter. So I'll leave you
there in this video, I've left you just saying, OK,
so the closeness to the sun does not dictate what
season we are in. And so you're saying,
what is the reason? And what we'll see
in the next video, the reason is the
tilt of the axis of the earth, the rotational
axis of the earth.