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  • Greetings from the Northern Hemisphere! ... where in a couple of weeks, summer will officially

  • begin, while our friends in Sydney, Johannesburg and Buenos Aires are getting ready for winter.

  • But no matter where you are, or when you're watching this, you're never more than three

  • months away from a change of seasons.

  • And who can we thank for this?

  • The sun, partly. And the way Earth leans into it.

  • So the earth spins on its axis, at a fixed angle of about 23 ½ degrees.

  • And as the earth goes around the sun, once every 365.25 days, this axial tilt, or obliquity,

  • changes the exposure to the sun's radiation that different parts of the earth experience

  • at any given time.

  • So, starting in a couple of weeks, the Northern Hemisphere will tilt toward the sun, and people

  • will break out the iced tea and sunscreen, while the Southern Hemisphere will point away,

  • and begin bundling up for the relative darkness of winter.

  • BUT! Here's the thing. Lots of people think that the hemisphere that's tilted toward the

  • sun experiences summer because it's closer, and therefore hotter.

  • Which is just false.

  • In fact, those of you in the Northern Hemisphere might be surprised to know that Earth is actually

  • 5 million kilometers closer to the sun in January than it is in July.

  • So the difference doesn't have to do with your hemisphere's proximity to the sun. It

  • has to do with the angle of the sun's rays that it receives.

  • Let's say this piece of paper is Earth's surface. And this flashlight is the light of the sun.

  • When I shine it straight onto the paper at a right angle, you see a perfect illuminated

  • circle. But when I tilt the flashlight, that circle stretches into an ellipse.

  • So the same total amount of light is hitting the paper, but it's now more spread out, so

  • the light energy per square centimeter is reduced.

  • This effect, combined with the fact higher sun angles result in longer days, means that

  • more energy is heating your hemisphere in the summer, and less in the winter.

  • But if you're near the equator, you probably barely notice any seasons at all, since you

  • get roughly the same amount of sun year round.

  • The poles, on the other hand, feel the difference in a big way. That 23 ½ degree tilt ensures

  • they never point directly at the sun, but they still spend about half the year bathed

  • in daylight and the other half in near total darkness.

  • So the next time you're picking daffodils, getting tan, carving pumpkins, or building

  • snowmen, take a second to think about how you're leaning toward or away from the sun,

  • and enjoy the uniqueness of the season, if you get 'em.

  • Thanks for asking, and thanks especially to our Subbable subscribers who keep these answers

  • coming!

  • If you have a quick question, let us know on Facebook or Twitter or in the comments

  • below, and if you want to keep getting smarter with us, just go to YouTube.com/scishow and

  • subscribe!

Greetings from the Northern Hemisphere! ... where in a couple of weeks, summer will officially

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B1 中級

なぜ季節があるのか? (Why do we have seasons?)

  • 146 9
    Mark Mantell に公開 2021 年 01 月 14 日
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