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When I was a kid,

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my understanding of the seasons

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was that December and January were cold

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and covered with snow,

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April and May were bursting with flowers,

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July and August were hot and sunshiny,

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and September and October were a kaleidoscope of colorful leaves.

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It was just the way the world worked,

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and it was magical.

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If you had told me back then

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that one-third of Earth's population

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had never seen snow

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

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I would have thought you were crazy.

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But in reality, seasonal change with four distinct seasons

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only happens in two regions on the planet.

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And, even in those two,

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the seasons are reversed.

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But why?

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A lot of people have heard of an astronomer

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called Johannes Kepler

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and how he proved that planetary orbits are elliptical

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

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It was a big deal when he figured this out

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several hundred years ago.

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His discovery solved a lot of mathematical problems

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that astronomers were having

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with planetary orbit measurements.

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

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those pictures in our science books,

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on TV, and in the movies

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give an exaggerated impression

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of how elongated our orbit is.

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

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

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even though it doesn't look like one,

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

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it means that our distance from the sun

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does change through the year.

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Ah-ha!

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

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Well, no, not so fast.

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The Earth is actually closer to the sun

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in January than we are in July

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by 5 million kilometers.

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January is smack-dab in the middle

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of the coldest season of the year

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for those of us up north.

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Still not convinced?

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How about this:

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Summer and winter occur simultaneously

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on the surface of our planet.

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When it's winter in Connecticut,

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it's summer in New Zealand.

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

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what else could it be?

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Well, we need to also need to know

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that the Earth doesn't sit straight up.

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It actually tilts.

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And that axial tilt of the Earth

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is one of the main reasons for the seasons.

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The Earth spins on an axis

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that's tilted 23.5 degrees from vertical.

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At the same time, the Earth revolves around the sun

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

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Together with the tilt,

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the spinning and revolving causes the number

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of hours of daylight in a region to change

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as the year goes by,

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with more hours in summer

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and fewer in winter.

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

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

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So, in the summer,

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

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like Hartford, Connecticut,

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will get 15 hours of daylight each day

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and 9 hours of darkness.

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It warms up for longer than it cools.

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This happens day after day,

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so there is an overall warming effect.

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Remember this fact for later!

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In the winter, the opposite happens.

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There are many more hours of cooling time

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than warming time,

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

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The interesting thing is, as you move north,

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the number of daylight hours in summer increases.

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

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

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In fact, in the summertime at the North Pole,

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the sun never sets.

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

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

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If daylight hours were the only thing

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that determined average temperature,

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wouldn't the North Pole be the hottest place

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on Earth in northern summer

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because it receives 24 hours of daylight

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in the months surrounding the summer solstice?

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But it's the North Pole.

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There's still icebergs in the water

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and snow on the ground.

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So, what's going on?

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The Earth is a sphere

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and so the amount of solar energy an area receives

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

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which, as you know, changes during the day

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between sunrise and sunset.

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But, the maximum height also changes during the year,

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with the greatest solar height during the summer months

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and highest of all at noon on the summer solstice,

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which is June 21st in the northern hemisphere

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and December 21st in the southern hemisphere.

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This is because as the Earth revolves,

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the northern hemisphere ends up tilted away

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from the sun in the winter

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and toward the sun in summer,

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which puts the sun more directly overhead

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for longer amounts of time.

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Remember those increased summer time daylight hours?

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And solar energy per square kilometer increases

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as the sun gets higher in the sky.

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So, when the sun's at an angle,

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the amount of energy delivered

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to each square of the sunlit area is less.

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Therefore, even though the North Pole is getting 24 hours

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of daylight to warm up,

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the sunlight it receives is very spread out

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and delivers less energy than a place further south,

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where the sun is higher in the sky

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because it's more tilted toward the sun.

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

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It was cooling down without any sunlight at all

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for 6 months straight.

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So, as the seasons change, wherever you are,

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

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but the astronomical complexity

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that brings them to you.