The Earth's Shadow Rises
Chosen NASA Astronomy Picture of the Day - Clink to Link

Just before sunset, a  flat, dark blue band rises up from the eastern horizon. This is the Earth's shadow. There is a pinkish antitwilight arch, above it.  When the sun sets, the boundary between the Earth's shadow and the antitwilight arch rises in the sky and becomes more difficult to see, until it eventually disappears and blends in with the night sky.

The Earth's shadow is visible because it is cast upon our own atmosphere. The antitwilight's color  originate in backscattering by the  thick lower atmosphere, and is illuminated by direct (reddened) sunlight. But as the twilight progresses,  the lower shadow-boundry of the atmosphere rises.  The shadow, now in a thinner part of the atmosphere, scatters less red light and a point is reached where the strong blue light  becomes the main source of light reaching our eyes.
 

This CCD image was taken with a Nikon D100 camera and Nikkor 35mm f/2 lens.  The exposure was 1/30th second at f/2.8.  Taken from my driveway in the coastal foothills above Santa Cruz, California.  The Moon, not quite full, rises in the East.
 

HOME