Earth's shadow

The Earth's shadow or Earth shadow (also sometimes known as the dark segment) is the shadow that the Earth itself casts on its atmosphere. This shadow is often visible from the surface of the Earth, as a dark band in the sky near the horizon. This atmospheric phenomenon can sometimes be seen twice a day, around the times of sunset and sunrise.

Whereas the phenomenon of night (a function of being in the shadow of the Earth) is familiar, the effect of the Earth's shadow on the atmosphere is quite often visible in the sky, but often goes unrecognized. This shadow is visible to observers as it falls on the atmosphere of the Earth during the twilight hours. When the weather conditions and the observer's viewing point permit a clear sight of the horizon, the shadow can be seen as a dark blue or greyish-blue band.

Assuming the sky is clear, the Earth's shadow is visible in the opposite half of the sky to the sunset or sunrise, and is seen right above the horizon as a dark blue band. A related phenomenon is the "Belt of Venus" or "anti-twilight arch" (sometimes written as "antitwilight arch"), a pink band that is visible above the dark blue of the Earth's shadow, in the same part of the sky. No defined line divides the Earth's shadow and the Belt of Venus; one colored band blends into the other in the sky.

The Earth's shadow (as it is cast onto the atmosphere) can be observed during the twilight hours, assuming the sky is clear and the horizon is relatively unobstructed. At sunset the Earth's shadow is visible opposite the sunset in the eastern sky, just above the horizon. The shadow shows as a dark blue band that stretches over 180° of the horizon. It is most noticeable at the antisolar point, exactly opposite the sunset.

At sunrise, the Earth's shadow is seen in a similar way, but in the western sky. The Earth's shadow is best observed when there is a low horizon (such as over the sea), and when the sky conditions are very clear. In addition, the higher up an observer is standing to view the horizon, the sharper the shadow appears.

At sunrise, the Earth's shadow can be seen to set as the sun itself rises, and at sunset, the Earth's shadow rises as the sun sets.

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Wikipedia