The Reason for the Seasons
One of the objectives from our lesson is to understand the role of the changing Sun angle and the
tilt of the Earth on its axis, and the roles that those play in the changing of the seasons. And so to
look at that, we're going to start with a virtual flashlight. And we have a situation here, where our
virtual flashlight is pointed directly at an object, so the light comes in perpendicularly and strikes
the object.
You can see that the light is fairly bright, it's spread over a small area, so the amount of light per
unit area is high, and the light is intense as it strikes the object. But if we change the angle of our
flashlight and make the light come in at a lower angle and strike the surface, the light now gets
spread out over a larger area. So the amount of light per unit area is now lower, and the light is
dimmer on the surface, it's not as intense. So even though the same amount of light is coming out
of the flashlight, it's dimmer on the surface, because of the lower angle that is striking the
surface.
You could try that experiment at home with a real flashlight, if you wanted. But in a nutshell,
you've seen the reason for the change of seasons. Earth is tilted on its axis at a 23.5-degree angle.
And as it revolves around the Sun, the location that gets the direct beam of light, when it's
coming in perpendicularly, that location changes throughout the year.
It's always between 23.5 degrees north latitude and 23.5 degrees south latitude. For example,
over the equator, the equator gets the direct beam of light twice a year on the spring equinox and
the autumnal equinox. And during the other part of the year, that direct beam is over other
various locations between 23.5 north and 23.5 south.
To better see how the changing Sun angle impacts the seasons, we have an animation here of the
Earth revolving around the Sun, and the Earth is tilted on its axis at the 23.5-degree angle. And
the seasonal designations here are for the Northern Hemisphere. So we can see the situation in
the Northern Hemisphere and why it's warmer in Northern Hemispheric summer.
We can see here that when it's summer in the Northern Hemisphere, the Northern Hemisphere is
tilted toward the Sun. So the Sun's light is striking the surface more directly, so the heating
power of the Sun is higher in the Northern Hemisphere during summer. Meanwhile, in the
Southern Hemisphere, that hemisphere is tilted away from the Sun. And like it's spread out over
a larger area, the Sunlight is coming in at a lower angle. And so the heating power of the Sun is
reduced in the Southern Hemisphere. That's why the Southern Hemisphere's having its winter,
while the Northern Hemisphere has its summer.
We can check out the situation, which is just the opposite then in the Northern Hemispheric
winter. During the Northern Hemisphere's winter, the Sun angle is low in the Northern
Hemisphere. It's coming in at a lower angle, and the light is spreading out over a larger area, so
the heating power of the Sun is reduced, because the Northern Hemisphere is tilted away from
the Sun, and the light is coming in at a low angle.
Meanwhile, the Southern Hemisphere is pointed more directly at the Sun. And so the Sun is
coming in more directly, the light is coming in more directly. And the heating power of the Sun
is stronger in the Southern Hemisphere. And that's why when it's winter in the Northern
Hemisphere, it's summer and warmer in the Southern Hemisphere.
With this basic understanding, then, we can ask ourselves a couple of interesting questions, like
what would happen if we changed the angle of the Earth's tilt, to say, 50 degrees, so it's much
more tilted. Well, that would actually result in more extreme seasonal changes, because, say, we
had a 50-degree angle of tilt here, the Northern Hemisphere during winter would be tilted even
farther away from the Sun, and the Sun angle would be even lower, and the sunlight would be
spread over a much larger area, so the heating power of the Sun would be reduced even further,
making winter in the Northern Hemisphere even colder.
Meanwhile, in the Southern Hemisphere, the sunlight would be coming in even more directly,
and it would be even hotter. So you'd have more extreme seasons if you increased the angle of
the Earth's tilt. But if you reduced it all the way to zero, say, the Earth had no tilt on its axis, then
seasonal variation would go away completely, because the angle that the Sun's light was coming
in to the Earth would not change throughout the year, and so we wouldn't have any changes to
our seasons.