Earth's shadow

This article is about the shadow the Earth casts on its atmosphere as viewed from the Earth. For information about how it (or any other shadow) extends in space, see Umbra.
Earth's shadow (blue) and the Belt of Venus (pink) at dawn, seen above the Pacific Ocean (blue-grey), looking west from Twin Peaks, San Francisco

Earth's shadow (or Earth shadow) is the shadow that Earth itself casts through its atmosphere and into outer space, toward the antisolar point. During the twilight period (both early dusk and late dawn), the shadow's visible fringe – sometimes called the dark segment or twilight wedge[1] – appears as a dark and diffuse band just above the horizon, most distinct when the sky is clear.

Since the angular sizes of the Sun and the Moon, visible from the surface of the Earth, are almost the same, the ratio of the length of the Earth's shadow to the distance between the Earth and the Moon will be almost equal to the ratio of the sizes of the Earth and the Moon. Since Earth's diameter is 3.7 times the Moon's, the length of the planet's umbra is correspondingly 3.7 times the average distance from the Moon to the Earth: roughly 1,400,000 km (870,000 mi). The width of the Earth's shadow at the distance of the lunar orbit is approximately 9000 km (~ 2.6 lunar diameters), which allows people of the Earth to observe total lunar eclipses.[2]

  1. ^ "Twilight wedge". www.weatherscapes.com.
  2. ^ Pogge, Richard. "Lecture 9: Eclipses of the Sun & Moon". Astronomy 161: An Introduction to Solar System Astronomy. Ohio State University. Retrieved July 16, 2015.