penumbra

[pi-nuhm-bruh]
noun, plural penumbrae [pi-nuhm-bree] , penumbras.
1.
Astronomy.
a.
the partial or imperfect shadow outside the complete shadow of an opaque body, as a planet, where the light from the source of illumination is only partly cut off. Compare umbra ( def 3a ).
b.
the grayish marginal portion of a sunspot. Compare umbra ( def 3b ).
2.
a shadowy, indefinite, or marginal area.

Origin:
1660–70; < Neo-Latin, equivalent to Latin paen- pen- + umbra shade

penumbral, penumbrous, adjective
Dictionary.com Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
penumbra (pɪˈnʌmbrə)
 
n , pl -brae, -bras
1.  a fringe region of half shadow resulting from the partial obstruction of light by an opaque object
2.  astronomy the lighter and outer region of a sunspot
3.  painting the point or area in which light and shade blend
 
[C17: via New Latin from Latin paene almost + umbra shadow]
 
pe'numbral
 
adj
 
pe'numbrous
 
adj

Collins English Dictionary - Complete & Unabridged 10th Edition
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Etymonline
Word Origin & History

penumbra
1666, from Mod.L. penumbra "partial shadow outside the complete shadow of an eclipse," coined 1604 by Kepler from L. pæne "almost" + umbra "shadow" (see umbrage).
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Science Dictionary
penumbra   (pĭ-nŭm'brə)  Pronunciation Key 
Plural penumbras or penumbrae (pĭ-nŭm'brē)
  1. A partial shadow between regions of full shadow (the umbra) and full illumination, especially as cast by Earth, the Moon, or another body during an eclipse. During a partial lunar eclipse, a portion of the Moon's disk remains within the penumbra of Earth's shadow while the rest is darkened by the umbra. See Note at eclipse.

  2. The grayish outer part of a sunspot. Compare umbra.


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

penumbra

(from Latin paene, "almost"; umbra, "shadow"), in astronomy, the outer part of a conical shadow, cast by a celestial body, where the light from the Sun is partially blocked-as compared to the umbra (q.v.), the shadow's darkest, central part, where the light is totally excluded. The definition of the term may be extended to include the partial shadow of any large source of light or electromagnetic radiation. An observer in the penumbra sees the Sun (or other light source) partially obscured. The term also is used for the outer portion of a sunspot.

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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Example sentences
But they have a penumbra of agents, lawyers and image-makers.
And perhaps dyscalculia will collect a penumbra of dubious cases around it, as dyslexia has.
Bloggers and pop musicians operate in a penumbra of semi-illegality.
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