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penumbra

[pi-nuhm-bruh] /pɪˈnʌm brə/
noun, plural penumbrae
[pi-nuhm-bree] /pɪˈnʌm bri/ (Show IPA),
penumbras.
1.
Astronomy.
  1. 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).
  2. the grayish marginal portion of a sunspot.
    Compare umbra (def 3b).
2.
a shadowy, indefinite, or marginal area.
Origin
1660-1670
1660-70; < New Latin, equivalent to Latin paen- pen- + umbra shade
Related forms
penumbral, penumbrous, adjective
Dictionary.com Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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British Dictionary definitions for penumbral

penumbra

/pɪˈnʌmbrə/
noun (pl) -brae (-briː), -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
Compare umbra
Derived Forms
penumbral, penumbrous, adjective
Word Origin
C17: via New Latin from Latin paene almost + umbra shadow
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition
© William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
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Word Origin and History for penumbral

penumbra

n.

1660s, from Modern Latin penumbra "partial shadow outside the complete shadow of an eclipse," coined 1604 by Kepler from Latin pæne "almost" + umbra "shadow" (see umbrage). Related: Penumbral.

Online Etymology Dictionary, © 2010 Douglas Harper
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penumbral in Science
penumbra
  (pĭ-nŭm'brə)   
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 Article for penumbral

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.

Learn more about penumbra with a free trial on Britannica.com
Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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