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penumbra

[ pi-nuhm-bruh ]
/ pɪˈnʌm brə /
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noun, plural pe·num·brae [pi-nuhm-bree], /pɪˈnʌm bri/, pe·num·bras.

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).
a shadowy, indefinite, or marginal area.

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Origin of penumbra

First recorded in 1660–65; from New Latin penumbra (coined by Johann Kepler in 1604), equivalent to Latin paen- prefix meaning “almost” + umbra “shade”; see pen-, umbra
pe·num·bral, pe·num·brous, adjective
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2021

British Dictionary definitions for penumbra

penumbra
/ (pɪˈnʌmbrə) /

noun plural -brae (-briː) or -bras

a fringe region of half shadow resulting from the partial obstruction of light by an opaque object
astronomy the lighter and outer region of a sunspot
painting the point or area in which light and shade blend
Compare umbra
penumbral or penumbrous, adjective
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 Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Scientific definitions for penumbra

penumbra
[ pĭ-nŭmbrə ]

Plural penumbras penumbrae (pĭ-nŭmbrē)

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.
The grayish outer part of a sunspot. Compare umbra.
The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
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