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Science & Environment

Explaining The Celestial Logistics Of The Winter Solstice


Tonight is the winter solstice, the longest night of the year. I always thought that meant the sun sets earlier tonight than any other night. Turns out I'm wrong. And to explain why, astrophysicist and NPR blogger Adam Frank is with us. Adam, what am I not getting?

ADAM FRANK: Well, basically the problem is the mismatch between what we might call technological time and astronomical time, so...

SHAPIRO: ...So my watch is not perfectly correlated to what's happening in the cosmos.

FRANK: That's exactly it. And in particular, as the Earth moves around the sun in its orbit, it's not a perfect circle. So it's actually kind of a squashed circle called an ellipse. When the Earth gets closer to the sun, it's actually moving faster in its orbit than when it's further away. And it's that sort of mismatch, that extra speed, that sort of causes the technological time and the astronomical time to drift from each other.

SHAPIRO: OK. Well, why do we actually have a solstice in the first place? And do days get longer and shorter over the course of the year?

FRANK: So the reason is - basically, it's about spinning things. The same reason we have seasons is the same reason why a good pass in football is a tight spiral. So the Earth is spinning on its axis, and spinning things don't like to change their direction. So as the Earth goes around the sun, the tilt of our spin axis is about 23 degrees or so. What that really basically means is when the Earth is on one side of the sun, the tilt in the summer will be pointed towards the sun. So the sun is directly overhead, the sun feels hotter. Six months later, the axis tilted away from the sun. And of course, then the sun is lower in the sky, and the sunlight hits us at a shallow angle and it feels cooler.

SHAPIRO: OK, so it's all about the tilt. If I go to a far-away planet in a different solar system, maybe even in a different galaxy, will there be seasons? Does every planet have a tilt?

FRANK: Well, it all depends. So Mars, for example, has a tilt that's similar to ours and it has pretty strong seasons. And as we go and look at planets, we've been discovering all these planets around other stars in the galaxy. And what we're finding is this wide variety of conditions. And in fact, some of the planets actually have their spins locked into their year, so that actually they don't have seasons at all. The - for a planet like that, the same face of the planet would always be facing the star. So basically, if you were directly under the star, the sun wouldn't move at all. So you know how some people say it's always happy hour somewhere on the Earth? Well, on those planets it would always be happy hour in the same place on the planet.

SHAPIRO: That's NPR blogger and astrophysicist Adam Frank. Happy solstice, Adam.

FRANK: And happy solstice to you too, Ari. Transcript provided by NPR, Copyright NPR.