Reasons for the seasons - Rebecca Kaplan

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Why do some regions experience full-time heat while others are reckoning with frigid temperatures and snow? And why are the seasons reversed in the two hemispheres? Rebecca Kaplan explains how the shape of the Earth's orbit around the Sun and the Earth's tilt on its axis affect the amount of sunlight each region receives.

Lesson by Rebecca Kaplan, animation by Marc Christoforidis.

When I was a kid,

my understanding of the seasons

was that December and January were cold

and covered with snow,

April and May were bursting with flowers,

July and August were hot and sunshiny,

and September and October were a kaleidoscope of colorful leaves.

It was just the way the world worked,

and it was magical.

If you had told me back then

that one-third of Earth's population

had never seen snow

or that July 4th was most definitely not a beach day,

I would have thought you were crazy.

But in reality, seasonal change with four distinct seasons

only happens in two regions on the planet.

And, even in those two,

the seasons are reversed.

But why?

A lot of people have heard of an astronomer

called Johannes Kepler

and how he proved that planetary orbits are elliptical

and that the sun is not at the center of the orbit.

It was a big deal when he figured this out

several hundred years ago.

His discovery solved a lot of mathematical problems

that astronomers were having

with planetary orbit measurements.

While it's true that our orbit's not perfectly circular,

those pictures in our science books,

on TV, and in the movies

give an exaggerated impression

of how elongated our orbit is.

In fact, Earth's orbit is very nearly a perfect circle.

However, because Earth's orbit is technically an ellipse,

even though it doesn't look like one,

and the sun isn't quite exactly at the center,

it means that our distance from the sun

does change through the year.

Ah-ha!

So, winter happens when the Earth is further away from the sun!

Well, no, not so fast.

The Earth is actually closer to the sun

in January than we are in July

by 5 million kilometers.

January is smack-dab in the middle

of the coldest season of the year

for those of us up north.

Still not convinced?

How about this:

Summer and winter occur simultaneously

on the surface of our planet.

When it's winter in Connecticut,

it's summer in New Zealand.

So, if it's not the distance from the sun,

what else could it be?

Well, we need to also need to know

that the Earth doesn't sit straight up.

It actually tilts.

And that axial tilt of the Earth

is one of the main reasons for the seasons.

The Earth spins on an axis

that's tilted 23.5 degrees from vertical.

At the same time, the Earth revolves around the sun

with the axis always pointing in the same direction in space.

Together with the tilt,

the spinning and revolving causes the number

of hours of daylight in a region to change

as the year goes by,

with more hours in summer

and fewer in winter.

So, when the sun is shining on the Earth, it warms up.

After the sun sets, it has time to cool down.

So, in the summer,

any location that's about 40 degrees north of the equator,

like Hartford, Connecticut,

will get 15 hours of daylight each day

and 9 hours of darkness.

It warms up for longer than it cools.

This happens day after day,

so there is an overall warming effect.

Remember this fact for later!

In the winter, the opposite happens.

There are many more hours of cooling time

than warming time,

and day after day, this results in a cooling effect.

The interesting thing is, as you move north,

the number of daylight hours in summer increases.

So, Juneau, Alaska would get about 19 hours of daylight

on the same summer day that Tallahassee, Florida gets about 14.

In fact, in the summertime at the North Pole,

the sun never sets.

OK, then, it's all about daylight hours, I've got it!

Well, no, there's another important piece to this puzzle.

If daylight hours were the only thing

that determined average temperature,

wouldn't the North Pole be the hottest place

on Earth in northern summer

because it receives 24 hours of daylight

in the months surrounding the summer solstice?

But it's the North Pole.

There's still icebergs in the water

and snow on the ground.

So, what's going on?

The Earth is a sphere

and so the amount of solar energy an area receives

changes based on how high the sun is in the sky,

which, as you know, changes during the day

between sunrise and sunset.

But, the maximum height also changes during the year,

with the greatest solar height during the summer months

and highest of all at noon on the summer solstice,

which is June 21st in the northern hemisphere

and December 21st in the southern hemisphere.

This is because as the Earth revolves,

the northern hemisphere ends up tilted away

from the sun in the winter

and toward the sun in summer,

which puts the sun more directly overhead

for longer amounts of time.

Remember those increased summer time daylight hours?

And solar energy per square kilometer increases

as the sun gets higher in the sky.

So, when the sun's at an angle,

the amount of energy delivered

to each square of the sunlit area is less.

Therefore, even though the North Pole is getting 24 hours

of daylight to warm up,

the sunlight it receives is very spread out

and delivers less energy than a place further south,

where the sun is higher in the sky

because it's more tilted toward the sun.

Besides, the North Pole has a lot to make up for.

It was cooling down without any sunlight at all

for 6 months straight.

So, as the seasons change, wherever you are,

you can now appreciate not just the beauty of each new season

but the astronomical complexity

that brings them to you.