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umbra

 [uhm-bruh]   Origin

um·bra

   [uhm-bruh]
noun, plural -bras, -brae  [-bree] .
1.
shade; shadow.
2.
the invariable or characteristic accompaniment or companion of a person or thing.
3.
Astronomy.
a.
the complete or perfect shadow of an opaque body, as a planet, where the direct light from the source of illumination is completely cut off. Compare penumbra (def. 1a).
b.
the dark central portion of a sunspot. Compare penumbra (def. 1b).
4.
a phantom or shadowy apparition, as of someone or something not physically present; ghost; spectral image.
Origin:
1590–1600; < Latin: shade, shadow

Related forms
um·bral, adjective
Dictionary.com Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2012.
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Umbra is always a great word to know.
So is Sagittarius. Does it mean:
a zodiacal constellation between Scorpius and Capricorn also called the Archer
the brightness of a star in comparison with that of the sun: the luminosity of Sirius is expressed as 23 and indicates intrinsic brightness 23 times greater than the sun
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Collins
World English Dictionary
umbra (ˈʌmbrə)
 
—n , pl -brae, -bras
1.  a region of complete shadow resulting from the total obstruction of light by an opaque object, esp the shadow cast by the moon onto the earth during a solar eclipse
2.  the darker inner region of a sunspot
 
[C16: from Latin: shade, shadow]
 
'umbral
 
—adj

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Etymonline
Word Origin & History

umbra
1599, "phantom, ghost," a fig. use from L. umbra "shade, shadow" (see umbrage). Astronomical sense of "shadow cast by the earth or moon during an eclipse" is first recorded 1679. Meaning "an uninvited guest accompanying an invited one" is from 1696.
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Science Dictionary
umbra   (ŭm'brə)  Pronunciation Key 
Plural umbras or umbrae (ŭm'brē)

  1. The darkest part of a shadow, especially the cone-shaped region of full shadow cast by Earth, the Moon, or another body during an eclipse. In a full lunar eclipse, which generally lasts for one or two hours, the entire disk of the Moon is darkened as it passes through the umbra. During this period the Moon takes on a faint reddish glow due to illumination by a small amount of sunlight that is refracted through the Earth's atmosphere and bent toward the darkened Moon; the reddish tint is caused by the filtering out of blue wavelengths as the sunlight passes through the Earth's atmosphere, leaving only the longer wavelengths on the red end of the spectrum. See Note at eclipse.

  2. The dark central region of a sunspot. Compare penumbra.


The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Encyclopedia Britannica
Encyclopedia

umbra

that part of a shadow in which all light from a given source is excluded. The shadow from a point source of illumination is essentially all umbra, but that from a source of some size (as from the Sun) consists of a small umbra and a much larger partial shadow called the penumbra. Thus, in an eclipse of the Sun, the regions within the umbra experience a total eclipse and those within the penumbra, partial eclipse. The term is also used for the dark central portion of a sunspot.

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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