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   Mountain Shadow 

   


Mt. Rainier in Washington State casts an apparently triangular shadow onto the sky. Dale Ireland (site) took the image from the 14,400 ft summit at a sunset in August '78. "I think I also have a similar shot taken the next morning which would be interesting to compare. It is not common for climbers to actually spend the night at the summit of this mountain due to the unpredictable weather. A cloud cap can form even in otherwise clear weather and the cold is numbing. Even the toothpaste froze."Photo ęDale Ireland, shown with permission
  

Seen from their summits almost all mountain shadows look triangular regardless of the mountain's shape. This is a perspective effect. You are standing at the top edge of a long tunnel of shadowed air and looking along its length. The tunnel's cross section is the shape of the mountain but its "end" is so far away that it looks insignificant.  The finite size of the sun also causes the umbral (fully shaded) parts of the shadow to converge and eventually taper away. The tapering sets limits to the umbral length of shadows. That of the Earth is over a million miles. That of a high mountain can be two to three hundred miles. Triangular shadows are not seen from objects much smaller than mountains because their shadows are not long enough.