Sunsets are Quite Interesting

Contributed by
Nov 20, 2011
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There's a wonderful comedic quiz show in the UK called "QI" -- for "Quite Interesting" -- which is hosted by none other than Stephen Fry. The participants are comedians, and they're asked questions ranging over just about every topic you can imagine. The BBC recently uploaded a clip about which alert BA Bloggee Brett Warburton informed me. In it, Fry shows the contestants a video of the Sun setting, and asks them to ring in when they think the Sun has completely set. Here's the clip:

This is, in fact, correct! The Earth's air bends the image of the Sun upward, so we can still see the Sun even though it is physically below the horizon. If we didn't have air, daytime would be shorter. In fact, this effect works for sunrise as well, so we see the Sun rise before it's physically cleared the horizon.

And Stephen was correct in the amount too; the light is bent upward by just about the same size as the Sun, so when the lower limb of the Sun just kisses the horizon it's actually already set.

But it's a bit more complicated, of course. The amount of bending changes as the Sun dips lower, because we're seeing through more air the closer to the horizon we look. I've written about this before. In fact, I found a nice graph from NOAA showing the amount of bending:

The amount a ray of light is bent is shown on the vertical axis, and the distance from the horizon on the horizontal. The units are degrees, with 360° in a circle, of course, and the Sun's size in the sky is about 0.5°. Reading off the graph, you can see that right on the horizon, a ray of light is bent upward by about a half degree. In fact, the light from the top of the Sun is bent less than the light from the bottom, so this effectively pushes the bottom of the Sun up toward the top, squishing it! You've probably seen countless pictures of the Sun looking squashed on the horizon (the Moon, too -- I just posted about that yesterday, in fact!). Well, that's why.

Now, what the comic was saying about the roads in New Zealand didn't make much sense to me; the Sun gets low to the horizon everywhere at some point during the day (well, except maybe for the poles during their local summer, but they don't have well-traveled roads anyway), so glaring reflections are always a problem.

I am endlessly fascinated by phenomena like this in the sky, and I'm pleased that "QI" was able to explain it to so many people. Even if one of the comics didn't like it. But there you go: the Universe is under no obligation to make us like it. It does what it does, and you might as well enjoy it when you can.

Image credits: NOAA; NASA

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