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.