pen·​um·​bra | \pə-ˈnəm-brə \
plural penumbrae\ pə-​ˈnəm-​(ˌ)brē , -​ˌbrī \ or penumbras

Definition of penumbra 

1a : a space of partial illumination (as in an eclipse) between the perfect shadow on all sides and the full light

b : a shaded region surrounding the dark central portion of a sunspot

2 : a surrounding or adjoining region in which something exists in a lesser degree : fringe the seventeenth century lay in the penumbra of the middle ages— Edward Eggleston

3 : a body of rights held to be guaranteed by implication in a civil constitution the penumbra of the Bill of Rights

4 : something that covers, surrounds, or obscures : shroud a penumbra of secrecy a penumbra of somber dignity has descended over his reputation— James Atlas

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Other Words from penumbra

penumbral \ pə-​ˈnəm-​brəl \ adjective

Synonyms for penumbra


dusk, shade, shadiness, shadow, umbra

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Did You Know?

Every solar eclipse casts an umbra, the darker central area in which almost no light reaches the earth, and a penumbra, the area of partial shadow where part of the sun is still visible. Penumbra can thus be used to describe any "gray area" where things aren't all black and white. For example, the right to privacy falls under the penumbra of the U.S. Constitution; though it isn't specifically guaranteed there, the Supreme Court has held that it is implied, and thus that the government may not intrude into certain areas of a citizen's private life. Because its existence is still shadowy, however, the Court is still determining how much of an individual's life is protected by the right to privacy.

Examples of penumbra in a Sentence

the lunar eclipse began with a subtle darkening of the lunar surface as it passed within the Earth's penumbra a penumbra of despair fell over the doomed city

Recent Examples on the Web

First with Neil Gorsuch and now with Justice Kennedy’s successor, Donald Trump is putting a stop to ruling by penumbra. Daniel Henninger, WSJ, "Trump Blows Away a Penumbra," 4 July 2018 In the far-off penumbra of Trump’s legal problems that center on his attorney Michael Cohen’s involvement with various hush-money payments, a lot happened — some funny but some conceivably quite damning. Matthew Yglesias, Vox, "Mike Pompeo’s excellent adventure, Tammy Duckworth’s baby, and more.," 20 Apr. 2018 The celestial show technically begins at 2:51 a.m. PST, when the moon moves into the pale outer shadow of the Earth, known as the penumbra. Deborah Netburn,, "Look up! Tonight's total lunar eclipse is special in at least three ways," 30 Jan. 2018 Analyzing penumbras could prove to be a crucial step to avoiding sudden accidents. David Grossman, Popular Mechanics, "MIT Scientists Working on Seeing Around Corners," 24 Oct. 2017 The penumbra of respectable opinion is always shifting in one direction or another, according to the whims of the Acela corridor, but its movements are slow enough that those who are borne along with it experience a smooth ride. T.a. Frank, The Hive, "Clinton’s Memoir Lives Up to Its Name for All the Wrong Reasons," 13 Sep. 2017 The rest of the country, including Sacramento, will be in the partial shadow provided by the penumbra, experiencing a partial eclipse. Don Sweeney, sacbee, "Here’s what you need to know about today’s eclipse in Sacramento," 21 Aug. 2017 The International Space Station will pass through moon's penumbra shadow, giving the crew a partial view each time. Mahita Gajanan, Time, "How Astronauts on the International Space Station Are Watching the Solar Eclipse," 21 Aug. 2017 The penumbra will be thousands of miles wide, reaching from Canada to Mexico. Julie Washington,, "Solar eclipse 2017 Q&A: Sunglasses, schedule, photography, map (video)," 16 Aug. 2017

These example sentences are selected automatically from various online news sources to reflect current usage of the word 'penumbra.' Views expressed in the examples do not represent the opinion of Merriam-Webster or its editors. Send us feedback.

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First Known Use of penumbra

1665, in the meaning defined at sense 1a

History and Etymology for penumbra

New Latin, from Latin paene almost + umbra shadow — more at umbrage

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Time Traveler for penumbra

The first known use of penumbra was in 1665

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More Definitions for penumbra


pe·​num·​bra | \pə-ˈnəm-brə \
plural penumbrae\ -​(ˌ)brē , -​ˌbrī \ or penumbras

Medical Definition of penumbra 

: a blurred area in a radiograph at the edge of an anatomical structure

Other Words from penumbra

penumbral \ -​brəl \ adjective


pe·​num·​bra | \pi-ˈnəm-brə \
plural penumbras

Legal Definition of penumbra 

1 : an area within which distinction or resolution is difficult or uncertain the public-private penumbra

2 : an extension of protection, reach, application, or consideration especially : a body of rights held to be guaranteed by implication from other rights explicitly enumerated in the U.S. Constitution the First Amendment has a penumbra where privacy is protected from governmental intrusion Griswold v. Connecticut, 381 U.S. 479 (1965) — see also Griswold v. Connecticut

Other Words from penumbra

penumbral \ -​brəl \ adjective

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More from Merriam-Webster on penumbra

Thesaurus: All synonyms and antonyms for penumbra

Spanish Central: Translation of penumbra Encyclopedia article about penumbra

Comments on penumbra

What made you want to look up penumbra? Please tell us where you read or heard it (including the quote, if possible).


something that serves to warn or remind

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