Earth's shadow
The Earth's shadow or Earth shadow (also sometimes known as the dark segment) are names for the shadow that the Earth itself casts on its atmosphere. That shadow is often visible near the horizon from the surface of the Earth.
Whereas the phenomenon of night (a function of being in the shadow of the Earth) is very familiar to all, the Earth's shadow on the atmosphere is quite often visible in the sky, and yet 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 permit a clear view of the horizon, the shadow can be seen as a dark blue or greyish-blue band in the sky. This effect can sometimes be seen twice a day, around the times of sunset and sunrise.
When 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.
Appearance
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 visible. 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 180º of the horizon.[1][2] 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.[1][2]
At sunrise, the Earth's shadow can be seen to set as the sun itelf rises, and at sunset, the Earth's shadow rises as the sun sets.[1]
Belt of Venus
A pink band is visible in the twilight sky just above the dark blue band of the Earth's shadow. This pink (or orange or purple) band is called the "anti-twilight arch" or "Belt of Venus". The name "Belt of Venus" is not connected with the planet Venus; the Belt of Venus is part of Earth's upper atmosphere which is illuminated by the setting or rising Sun. It is visible either after the sun ceases to be visible (at sunset) or before the sun becomes visible (at sunrise). [1][2]
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 scattered in the lower atmosphere on the other side of the sky from the sun, 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 Raleigh scattering from air molecules. Eventually, both the Earth's shadow and the Belt of Venus dissolve into the darkness of the night sky.[2]
A note on the color of lunar eclipses
The Earth's shadow or umbra is as curved as the planet Earth is, and extends hundreds of thousands of miles 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), gradually turning 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 still reach the Moon, even when the lunar eclipse is total; however this is light that has been refracted or bent as it passes through the Earth's atmosphere. The light 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. [3]
See also
References
- ^ a b c d Les Cowley. "Earth's shadow". www.atoptics.co.uk.
- ^ a b c d "What causes layers in the sunrise and sunset?". earthsky.org.
- ^ Color and light in nature. Cambridge University Press; 2 edition. July 2001. p. 38,39. ISBN 978-0521775045.
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