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 and Belt of Venus at sunrise, seen over a horizon where the sea meets the sky, looking west from Twin Peaks, San Francisco. Note: the lowest blue-grey area is not the sky but the surface of the Pacific Ocean.

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. 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 very familiar to all, the effect of 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 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. There is no clear dividing line between the Earth's shadow and the Belt of Venus; one colored band shades into the other in the sky.

Appearance

Earth's shadow and Belt of Venus at sunset, looking east from the Marin Headlands just north of San Francisco. Note: there is a thin greyish cloud layer partially obscuring the horizon in this image.

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 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

Main article: Belt of Venus

Full moon rising, as seen through the Belt of Venus. A very small part of the Earth's shadow (dark blue) is also visible in this image, but the horizon here is too high to see more of the Earth's shadow.

In the right viewing conditions, a pink (or orange or purple) band is visible in the twilight sky just above the dark blue band of the Earth's shadow. This pink 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]

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.^[2]

Color of lunar eclipses

Main article: Lunar eclipse

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).

An image taken during a total lunar eclipse on Feb 9th 2009 shows the reddish light falling on the moon's surface

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.^[3]

References

1. ^ ^{a b c d} Les Cowley. "Earth's shadow". www.atoptics.co.uk. http://www.atoptics.co.uk/atoptics/earshad.htm. 
2. ^ ^{a b c d} "What causes layers in the sunrise and sunset?". earthsky.org. http://www.webexhibits.org/causesofcolor/14E.html. 
3. ^ David K. Lynch, William Charles Livingston (July 2001). Color and light in nature. Cambridge University Press; 2 edition. p. 38,39. ISBN 978-0521775045. http://books.google.com/books?id=4Abp5FdhskAC&pg=PA38&lpg=PA38&dq=earth+shadow+color+and+light+in+nature&source=bl&ots=bN3UHSoym0&sig=DIeWBajGeM4ecoPW1KKNmhLyaV0&hl=en&ei=pFe8TLjoLJGqsAPcsemzDw&sa=X&oi=book_result&ct=result&resnum=1&ved=0CBIQ6AEwAA#v=onepage&q=earth%20shadow%20color%20and%20light%20in%20nature&f=false. 

External links

• Definition of "dark segment"
• Image showing a much larger segment of the sky with dark segment and Belt of Venus