World Library  
Flag as Inappropriate
Email this Article

Earth's shadow

Article Id: WHEBN0029247528
Reproduction Date:

Title: Earth's shadow  
Author: World Heritage Encyclopedia
Language: English
Subject: Afterglow, Umbra, penumbra and antumbra, April 2014 lunar eclipse, Gegenschein, Index of physics articles (E)
Collection: Atmospheric Optical Phenomena, Shading, Shadows
Publisher: World Heritage Encyclopedia
Publication
Date:
 

Earth's shadow

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. No defined line divides the Earth's shadow and the Belt of Venus; one colored band blends into the other in the sky.

Contents

  • Appearance 1
  • Belt of Venus 2
    • Color 2.1
  • Color of lunar eclipses 3
  • References 4
  • External links 5

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 180° 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 itself rises, and at sunset, the Earth's shadow rises as the sun sets.[1]

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

A total lunar eclipse on February 9, 2009, shows the reddish light falling on the moon's surface.

The Earth's shadow or umbra is as curved as the planet Earth is, and it 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, 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
  2. ^ a b c d
  3. ^

External links

  • Definition of "dark segment"
  • Image showing a much larger segment of the sky with dark segment and Belt of Venus
This article was sourced from Creative Commons Attribution-ShareAlike License; additional terms may apply. World Heritage Encyclopedia content is assembled from numerous content providers, Open Access Publishing, and in compliance with The Fair Access to Science and Technology Research Act (FASTR), Wikimedia Foundation, Inc., Public Library of Science, The Encyclopedia of Life, Open Book Publishers (OBP), PubMed, U.S. National Library of Medicine, National Center for Biotechnology Information, U.S. National Library of Medicine, National Institutes of Health (NIH), U.S. Department of Health & Human Services, and USA.gov, which sources content from all federal, state, local, tribal, and territorial government publication portals (.gov, .mil, .edu). Funding for USA.gov and content contributors is made possible from the U.S. Congress, E-Government Act of 2002.
 
Crowd sourced content that is contributed to World Heritage Encyclopedia is peer reviewed and edited by our editorial staff to ensure quality scholarly research articles.
 
By using this site, you agree to the Terms of Use and Privacy Policy. World Heritage Encyclopedia™ is a registered trademark of the World Public Library Association, a non-profit organization.
 


Copyright © World Library Foundation. All rights reserved. eBooks from Project Gutenberg are sponsored by the World Library Foundation,
a 501c(4) Member's Support Non-Profit Organization, and is NOT affiliated with any governmental agency or department.