Forget the Green Lantern: here’s the Green Flash

"The Earth’s atmosphere acts like a lens, bending light. Near the horizon this bending effect can be quite large, distorting objects when they rise or set (I explain this in detail in this post showing a similar picture of the squished Moon). Different wavelengths of light are bent by different amounts, so in extreme cases the colors from an object can be separated out, like the way a prism separates colors. "

http://blogs.discovermagazine.com/badastronomy/2011/05/03/forget-the-green-lantern-heres-the-green-flash/