Twilight Arch at Sunrise.
The earth casts its shadow onto the atmosphere at sunrise and sunset.
The earth, shown in brown, casts a shadow, shown in gray with a reddish edge, onto the atmosphere, shown in pastel cyan.
In the above photo taken at sunrise from the trail up the west side of Baboquivari Peak the dark band is the part of the atmosphere in complete shadow. Above it is a red band where the air is illuminated by sunlight that has passed through the lower atmosphere where the blue light has been scattered away leaving only the red to shine on the air. Above the red band the atmosphere is illuminated by full sunlight.
In a wide angle view the shadow can be seen to have an arc. Thus it is called the twilight arch.
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Scientific Explorations with Paul Doherty
6 Dec 99