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  • 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.

When I was a kid,

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B1 TED-Ed sun daylight summer earth north pole

【TED-Ed】Reasons for the seasons - Rebecca Kaplan

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    阿多賓 posted on 2014/05/01
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