How Everything Works
How Everything Works How Everything Works
 

QUESTIONS AND ANSWERS
 
Question 1241

I once saw a green sunrise. Can you explain this?
Apparently there are conditions in which green light from the sun is bent by the atmosphere so that it is visible first as the sun begins to rise above the horizon. Instead of seeing the yellow edge of the sun peaking up from behind the water or land, you see a green edge that lasts a second or two before being replaced by the usual yellow. This green flash is the result of refraction (bending of light) and dispersion (color-dependent light-speed) in air and is discussed in considerable detail at http://www.isc.tamu.edu/~astro/research/sandiego.html. According to the author of that site, Andrew Young, given a low enough horizon, which is the primary consideration, and clear air, which is also important, and a little optical aid, which helps a lot, one can certainly see green flashes at most sunsets.
         

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