Umbra of the Earth's Shadow
Umbra of the Earth’s Shadow
the part of space that is not penetrated by the direct rays of the sun, owing to the screening of the rays by the earth. The shape of the umbra is close to that of a circular cone. Since the distance of the earth from the sun varies in the course of the year, the length of the shadow cone varies slightly. On the average, the vertex of the cone is at a distance of 1.4 million km from the earth.
When the moon passes through the umbra of the earth’s shadow, a lunar eclipse is observed. Artificial earth satellites become invisible when they enter the umbra. In clear weather, the region of the earth’s atmosphere not illuminated by the direct rays of the sun can be observed at twilight in the part of the sky opposite the sun, which is below the horizon. The region has the appearance of a bluish gray dark segment bordered by a purplish band, which is called the antitwilight arch.