dipper@utastro.UUCP (Debbie Byrd) (07/04/85)
In a minute we'll tell you some reasons why the setting sun looks so very special. Stay with us. July 4: Illusions at Sunset If you're outside when the sun goes down today, take special notice of the sun itself. When the sun is low in the sky, we see it through a thicker mass of atmosphere than when it's overhead. The air scatters the sunlight -- blue light most, and red light least. When the sun is near the horizon, the thick atmosphere scatters away most of the blue light--but lets the red light travel straight through to our eyes. That's why the sun looks red when it's setting. The sun seen near the horizon also looks larger than usual -- same as the full moon does when it's near the horizon. This effect isn't caused by the atmosphere -- instead, it's a perceptual effect, possibly caused by the position of your eyes when you look at the sun straight on. But here's another atmospheric effect. The sun on the horizon can look slightly flattened -- or even broken in two. The greater thickness of atmosphere we look through when the sun is near the horizon causes the sun's light to be refracted, or bent. Refraction of light in the atmosphere is the same kind of effect that makes a spoon in a glass of water appear broken in two. The closer the sun is to the horizon, the more its light is refracted. Because of this, we actually see the sun for a few minutes before it rises and after it sets each day. When you next see the sun appear to touch the horizon, remember that the sun isn't really on the horizon when it appears to be. Instead, it's just below it. Script by Deborah Byrd. (c) Copyright 1984, 1985 McDonald Observatory, University of Texas at Austin