This is my fault, so I would like to clarify this point. It looks like that my answer was a bit too synthetic and the role of the atmosphere was not so clear. I include a (totally professional) sketch illustrating the phenomenon: The sky is not completely dark in the cone of shadow because of scattering from the atmosphere surrounding the cone, so it retains that orange-ish or blue-ish color, depending on the hour the photo was taken. The effect is even clearer in other pictures, like this one: In a certain way, we can say that the night sky itself is the result of the shadow of the Earth being cast on the sky (think about it).īasically, when you are standing on top of the mountain, you are seeing a portion of the sky in which it is already night. What is that dark blue band at the horizon? Why should a shadow be cast on something? That was the beginning and end of a day in the variable hour ancient calendars. It is also seen in the shadow of the earth as the sun rises on a clear sky, a deep blue separated often from the pink/orange of sunrise and sunset. To elaborate on "unlit air", as an answer to your "There don't appear to be any clouds or anything," air itself reflects light, as is evident at night when it is dark. Only the shadow's end carries much information about the mountain shape and it is so far away and in any event blurred by the 0.5º angular spread of the sun's rays that it is hardly visible You are standing at the top edge of the shadowed tunnel and looking out along its length which can be more than a hundred miles. Mountain shadows at sunrise and sunset are immensely long tunnels of unlit air, crepuscular rays in fact.įrom the summit, perspective effects nearly always make the shadow triangular regardless of the mountain's profile. Triangular shadows are not seen from objects much smaller than mountains because their shadows are not long enough. That of a high mountain can be two to three hundred miles. That of the Earth is over a million miles. The tapering sets limits to the umbral length of shadows. The finite size of the sun also causes the umbral (fully shaded) parts of the shadow to converge and eventually taper away.
The tunnel's cross section is the shape of the mountain but its "end" is so far away that it looks insignificant. You are standing at the top edge of a long tunnel of shadowed air and looking along its length.
Seen from their summits almost all mountain shadows look triangular regardless of the mountain's shape.
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