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Stauchy Blog » Blog Archive » The Physics of Sunset, Thur March 22 (JRS)

The Physics of Sunset, Thur March 22 (JRS)

We’ve learned a few things about the sunset, and it’s relatively easy to see them since everything happens so slowly. It all has to do with the refraction of the light caused by the Earth’s atmosphere.  For more, higher-res pictures, see the gallery: South Pole Sunset.


As you probably know, the fall equinox (or spring equinox in the northern hemisphere) near March 21. The equinox is the time of year when the sun’s position crosses the equator. Since we are at the Pole, if there were no atmosphere, it would be at this time that the sun would set. However, since the atmosphere refracts (bends) the light, we can still see the image of the sun for a few days after the equinox. The light is bent by about 0.6 degrees, which is about one diameter of the sun. That means that when you see the very bottom of the sun hit the horizon, the sun is actually already below the horizon, but the light is bent so that you can see it.Another phenomenon that is caused by the refraction of light is that the image of the sun gets squashed. You’ve probably noticed that the sun looks fatter and shorter (oblate) as it is setting. This is because the light from the bottom of the sun is traveling through more atmosphere and is getting bent more than the light from the top.

The last thing we learned more about is the “green flash”, which you may have heard of. This also has to do with refraction. That has to do with the prism effect. The amount that light is bent (refracted) depends on its wavelength (color). Blue light bends the most, while red light bends the least, with green being somewhere in the middle. That’s why we see red during sunset, because most of the blues and greens have been deflected by the atmosphere, while the red light has passed right through. Well, the actual image of the sun is slightly shifted by color. That is, the red image of the sun appears to be slightly lower than the green image of the sun, which is slightly lower than the blue image. But, the red image is much, much more intense than the green image (because much more of the green light gets scattered away by the atmosphere), so you can’t see the green image until the red image has set. So, at the moment that the red image of the sun goes entirely below the horizon, the green image can still be seen (the blue image can not really be seen because a vast majority of the blue light has been scattered by the atmosphere). This can only be seen on a very calm, clear day with a very crisp horizon. Mostly, it is observed over water. The unique part about being here is that the sunset lasts for days, and from what we’ve heard, it is possible to see the green flash for hours on end. But, it still requires a very calm, clear day, which we never really had during the sunset time frame. While looking through a telescope, I saw hints of green a couple of times, but it wasn’t really a clear image because of the thermal layering of the atmosphere (like looking at something far away on a hot day). Some people told us that they saw the green flash fairly clearly, but we never did. Oh well, perhaps we’ll see it at sunrise.-JRS



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2 Responses to “The Physics of Sunset, Thur March 22 (JRS)”

  1. joel Says:

    Hey you guys,

    Get back to work! You haven’t updated your blog in over a monthto . You’d better not be suffering from sun deprivation, cause sheeet, you got a long time to go. I had a dream about you two last night, Which prompted me see if you had anything of this sort. I hope everything’s going well. Sometimes I wish I was there not being responsible for the direction of my life, and unfortuantley my life is beginning to feel like a woody guthrie song. I wonder, is is fuckin-A cold there yet.

    Anyway, I just woke up and of course I don’t make any sense. Tell everyone whose name I don’t remember I said hi and I’m always thinking about you on the Dark Side: Neal, Leah, Emerys, Cryin Nick, Terri, Francie, Kevin. I can’t remember everyone of couse cause when I left the only thing I knew is that I wanted to get the hell out of there. See you guys someday.

    Joel (joel_wagner@hotmail.com)

  2. stauchy Says:

    Joel! What’s up, my man. Yeah, we’re constantly behind on our blog. We’ll get caught up sometime. It’s definitely fuckin-A cold, although right now we’re in the middle of a heat wave… it’s been in the -60’s. It got to -96 one evening, so that got everyone talking about the 300 club, but that’s the only time it’s gotten close. We’re still enjoying our time here, and we’re not really in any rush to leave. A few people have started counting down the days, but it’s way to early for that. Hope everything’s well with you. Take care!

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