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SYFY WIRE Bad Astronomy

Happy December Solstice!

By Phil Plait

Today, at 10:44 UTC (05:44 Eastern U.S. time), the Earth’s northern axis was pointed as far away from the Sun as it could be, or, if you prefer, the Earth’s southern axis was tipped toward the Sun as much as it could be*. We call that point the December solstice.

The Earth spins on its axis once per day, and orbits the Sun once per year. Over that time, the axis stays pretty much fixed — if you extend the northern axis into the sky, it points very close to the star Polaris (hence its name) all year ‘round.

But the Earth’s rotation axis is tipped a bit with respect to its orbital plane:

See? When the Earth is over to the right in the animation, the northern axis is pointed more or less in the Sun’s direction, and when it’s on the left it’s pointed away. In northern summer, near the summer solstice, the North Pole gets 24 hours a day of sunlight, and the South Pole gets 24 hours of darkness.

You can see this for yourself. Literally! Himawari 8 is a Japanese weather satellite that takes images of the Earth every 10 minutes. It’s in a geostationary orbit, which means it goes around the Earth once per day, so from its point of view it “hovers” over the same spot of the Earth. This animation shows the Earth as seen from Himawari 8 just yesterday (Dec. 20, 2016); watch carefully:

The satellite orbits at the same speed the Earth spins, so it always sees the same part of our planet; note Australia below center. Over the course of a day the Earth’s shadow sweeps over the surface, creating nighttime. Watch the video again, and pay attention to the North Pole, at the top of the frame: It always stays dark! If you’re at the North Pole, you’re in 24 hours of darkness; the Sun never rises.

Now look at the South Pole, at Antarctica. There, the shadow never quite reaches the bottom, and you can always see a bit of the austral continent lit. The Sun never sets there at this time of year!

Things are reversed at the June solstice, and you can see that for yourself as well in an amazing video that follows the Sun over the course of a day above the Arctic Circle.

So, if you live at the North Pole (ho ho ho) don’t despair: You’ve made it halfway through the long night, and you’ll get your first sunrise in just three months.

It’s funny how many different motions make up the clockwork of our planet, and how much they affect us, perhaps without us even knowing why. Unless you read my blog, of course. Then you’ll know.

And I haven’t even mentioned our orbit being an ellipse! Which reminds me: Next up: perihelion, on Jan. 4, 2017 at 14:18 UTC!

* 6.5 of one, half a baker’s dozen of the other.

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