Earth Shadow

Earth Shadow: Just before sunset, a low flat, dark blue band rises up from the eastern horizon. This is the earth shadow and it stretches for nearly 180°. It is bounded above by the pinkish anti-twilight arch and below by the horizon. The earth's shadow is best seen when the sky is clear and our line of sight is long. From a high elevation, the shadow appears sharper than it does from ground level. As the sun sets, the boundary between the earth shadow and the anti-twilight arch rises in the sky and becomes progressively less distinct. With the disappearance of the anti-twilight arch, the shadow blends smoothly with the deepening blue night sky. The earth shadow can be seen at twilight and sunrise most days, but it is not always blue. For a shadow to be seen, it must be cast upon something. The earth shadow is cast upon the atmosphere. Eventually the anti-twilight arch fades to blue. Its strong color and brightness near sunset originate in back scattering by the relatively thick lower atmosphere, being illuminated by direct (reddened) sunlight.