Why are mountain shadows always triangles?
I mean big mountain near daybreak or sunset. If you don't believe me, here are some pictures:
http://i125.photobucket.com/albums/p53/mkvispics/MountainShadowCaptioned.jpg
http://epod.usra.edu/blog/2007/12/mountain-shadow-in-southern-greece.html
http://www.alpinestateofmind.com/gallery/6496951_Gt5VK#534265336_vBbEZ
http://www.exo.net/~pauld/lectures/patternscostarica/patternsnature2004.htm (scroll down a little)
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My take is that this is similar to a picture of railroad tracks. ( http://farm3.static.flickr.com/2118/2203250246_af06cddc67.jpg )
Parallel lines when mapped on to a 3D sphere converge
Here, it is the light rays going around the mountain that appear to converge. And most of the details of the mountain get wiped out. The general shape of the mountain only magnifies this effect.
The convergence of the sun's rays at the horizon are called anticrepuscular rays. Here is a good picture of this effect: http://antwrp.gsfc.nasa.gov/apod/ap060917.html . The big difference is when on a mountain, you are looking down on the anticrepuscular rays, not up at them.