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Bending Sunlight

Week of July 12, 1999

Regular readers know that I am fascinated by atmospheric phenomena, including rainbows, sundogs, sun pillars, crepuscular rays and glories. These aren't strictly astronomy, of course, but what astronomer isn't interested in weather (and, for that matter, what meteorologist isn't interested in astronomy?)?

diagram of curved Earth Just last night I saw the sun set, and this morning I was up early enough to watch it rise. It made me think of another odd effect that most people aren't aware of. Like rainbows, glories and the like, it's due to refraction. As the sun sets, the light from the Sun is passing through more and more air. This is because the Earth is curved, and the atmosphere curves with it (see the diagram to the left, or click it to see the original at the Langley Research Center website).

The Earth's air does a lot of stuff to sunlight, including scattering the blue light out of it, making the sky look blue. This is also why the sun looks red at the horizon; the bluer light is being redirected into the air. Anyway, another effect is that the light from the Sun gets bent, or refracted, as it passes through air. This is the same effect that makes a straw in a glass of water look like it's bent. The more air the sunlight passes through, the more it's bent.

The Sun has a finite extent in the sky; that is, it has a definite size. Just at sunset (or sunrise), the top of the Sun is farther from the horizon than the bottom. The bottom part is going through more air, and is bent more! The light from the bottom gets bent up, towards the top, which makes the Sun look squashed. It may be an illusion that the Sun and Moon look bigger on the horizon, but it's no illusion that they look flattened. It's a real effect, and the air is behind it.

Incidentally, there is another weird thing associated with this. Since the light gets bent up, away from the horizon, you actually can see the Sun after it has physically set. Imagine there's no air: the Sun sets completely the instant the upper limb of the Sun is below the horizon. However, we have air, and the light gets bent upwards. The Sun is actually below the horizon, but you can still see it. Weird!

And another weird thing (I know a lot of weird stuff): the Sun sets at slightly different times because of your height. If you are higher up, the horizon appears lower, and the Sun sets a little later. Tall people see the Sun longer than short people! Don't believe me? I have done this myself at the beach: watch the Sun set (this only works for people on western coasts). When the top of it is about to dip below the horizon, squat down. Just as the Sun disappears, jump up as fast as you can. You'll see the Sun again! Sure, you'll look silly, but you'll be verifying an interesting scientific result. If someone asks you what you're doing, you can tell them in detail!

©2008 Phil Plait. All Rights Reserved.

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