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Through a Glass, Darkly

Week of August 3, 1998

I was driving home from work the other day, and the setting Sun was a bit of a nuisance. The last leg of my drive is almost due West, and so the Sun is always plaguing me, dancing in and out of my vision, leaving all sorts of afterimages on my retinae. Anyone who has ever driven in Washington DC knows that driving there is taking your life in your hands, and if you can't see the maniac trying to push you off the road you might as well close your eyes and hope for the best.

With that in mind, I decided to try to position myself on my seat so that the Sun would be behind the blue tinted part of my windshield, which starts at the top of the glass and stretches down about 15 centimeters. I was hoping the blue tinting would filter the Sun's light enough so that I could concentrate better on the road. The Sun was getting pretty low, and was already fairly red, as it does near sunset. I was surprised, then, when I finally got the Sun behind the blue tint and suddenly the Sun looked yellow!

I wasn't expecting that at all, but maybe I should have. The Sun puts out light of every color, but when they are blended they look white. Molecules in the air act like little pinball bumpers, scattering away some of those colors. Blue is the most easily affected, which is why the sky is blue: the blue light never makes it straight from the Sun to our eyes, but gets randomly scattered all over the sky. As the Sun sets further, green, yellow and sometimes even orange colors are scattered away, leaving only the red end of the spectrum to make it to our eyes, coloring the Sun a glowing red when it is on the horizon.

The blue filter in my windshield, however, blocks out all colors at the redder end of the spectrum. Yellow is in between red and blue in the spectrum, and so at sunset I had a prejudice that all the yellow was scattered away. That's not true; only most of it is, but some still remains. It is simply overpowered by the red light, so the Sun still looks red. However, the blue tinting in my windshield blocks red more the yellow, so it let the yellow through while stopping the red. So oddly, through my blue filter, the Sun suddenly looked like it does when it is high in the sky.

Most people don't think that astronomy is dangerous, but there I was, on a Washington DC road, staring at the setting Sun. I've seen people run off the road for less! They say Galileo went blind later in life from looking at the Sun through his telescope. Maybe I should consider myself lucky!

Oops! (October 17, 2000): Astronomer Andrew Young at San Diego State University pointed out to me that Galileo did not go blind from observing the Sun. He had glaucoma and cataracts. He actually did most of his observing when the Sun was low to the horizon, and it's far more safe to observe. I suppose this would make a good Bad page all by itself! I always wondered just why anyone would try to observe the Sun when it was high in the sky, especially someone as intelligent as Galileo. I need to question my own beliefs more often!

©2008 Phil Plait. All Rights Reserved.

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