Earth's shadow or Earth shadow is the shadow that Earth itself casts onto its atmosphere and into outer space, toward the antisolar point. During twilight (both early dusk and late dawn), the shadow's visible fringe (sometimes called the dark segment or twilight wedge) appears in a clear sky as a dark and diffused band low above the horizon.
Video Earth's shadow
Appearance
Earth's shadow cast onto the atmosphere can be viewed during the "civil" stage of twilight, assuming the sky is clear and the horizon is relatively unobstructed. The shadow's fringe appears as a dark bluish to purplish band that stretches over 180° of the horizon opposite the Sun, i.e. in the eastern sky at dusk and in the western sky at dawn. Before sunrise, Earth's shadow appears to recede as the Sun rises; after sunset, the shadow appears to rise as the Sun sets.
Earth's shadow is best seen when the horizon is low, such as over the sea, and when the sky conditions are clear. In addition, the higher the observer's elevation is to view the horizon, the sharper the shadow appears.
Maps Earth's shadow
Belt of Venus
A related phenomenon in the same part of the sky is the Belt of Venus, or anti-twilight arch, a pinkish band visible above the bluish shade of Earth's shadow, named after the planet Venus which, when visible, is typically located in this region 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 Belt of Venus is quite a different phenomenon from the afterglow, which appears in the geometrically opposite part of the sky.
Color
When the sun is near the horizon at sunset or sunrise, the light from the sun is red; this is because the light is reaching the observer through an especially thick layer of the atmosphere, which works as a filter, scattering all but the red light.
From the viewpoint of the observer, the red sunlight directly illuminates small particles in the lower atmosphere on the other side of the sky from the sun. The red light is backscattered to the observer, and that is why the Belt of Venus appears pink.
The lower the sunset sun descends, the less clearly distinguished the boundary between the Earth's shadow and Belt of Venus becomes. This is because now the setting sun illuminates a thinner part of the upper atmosphere. The red light is not scattered there because there are fewer particles, and the eye only sees the "normal" (usual) blue sky, which is due to Rayleigh scattering from air molecules. Eventually, both the Earth's shadow and the Belt of Venus dissolve into the darkness of the night sky.
Color of lunar eclipses
The Earth's shadow or umbra is as curved as the planet Earth is, and it extends 1.4 million kilometers into space (the antumbra, however, extends indefinitely). When the Sun, the Earth and the Moon are aligned perfectly (or almost perfectly), with the Earth in between the Sun and the Moon, the Earth's shadow falls onto the surface of the Moon which is facing the night side of the Earth, such that observers see the shadow gradually turn the bright full Moon dark, and then light again, creating a lunar eclipse.
During a lunar eclipse, a very small amount of light from the sun does however still reach the Moon, even when the lunar eclipse is total; this is light which has been refracted or bent as it passes through the Earth's atmosphere. This sunlight has been scattered by the dust in the Earth's atmosphere, and thus that light is red, in the same way that sunset and sunrise light is red. This weak red illumination is what causes the eclipsed Moon to be dimly reddish or copper-colored in appearance.
See also
- Brocken spectre, the apparently enormous and magnified shadow of an observer cast upon the upper surfaces of clouds opposite the sun
References
External links
- Definition of "dark segment"
- Image showing a much larger segment of the sky with dark segment and Belt of Venus
- Shadow of Earth, Belt of Venus as seen over Half Dome, Yosemite National Park, displayed in an interactive panorama. Scroll to the very bottom of the post to view, after all other Yosemite panoramas.
Source of the article : Wikipedia
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