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.
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