penumbra

American  

[pi-nuhm-bruh] / pɪˈnʌm brə /

noun

• penumbrae,
  plural
• penumbras
  plural

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.

   2. the grayish marginal portion of a sunspot.

2. a shadowy, indefinite, or marginal area.

   a penumbra of secrecy.

3. U.S. Law. a right or set of rights implicit but not expressed in the U.S. Constitution or Bill of Rights.

   The principle of the penumbra protects against an imperfect, or incomplete, enumeration of rights.

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penumbra British  

/ pɪˈnʌmbrə /

noun

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

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penumbra Scientific  

/ pĭ-nŭm′brə /

• penumbras
  plural

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.

2. See Note at eclipse

3. The grayish outer part of a sunspot.

4. Compare umbra

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Other Word Forms

Derived Forms

• penumbral
  adjective
• penumbrous
  adjective

Inflected Forms

Nouns

• penumbrae
  plural
• penumbras
  plural

Etymology

Origin of penumbra

First recorded in 1660–65; from New Latin penumbra (coined by Johannes Kepler in 1604), equivalent to Latin paen- prefix meaning “almost” + umbra “shade”; see pen-, umbra

Explanation

When you measure your shadow to calculate the angle of the sun in the sky, be sure to measure to the edges of your shadow, to the penumbra, the part of a shadow that is not as dark as the center. You're likely to come across penumbra most often in astronomy, as with an eclipse, where shadow is a defining feature. In a lunar eclipse, the edge of the earth's shadow — the part that isn't fully dark — is its penumbra. Sun spots also have a penumbra, the outer edge that's not quite as dark as the center. The word comes from the Latin root umbra, which means "shadow." The pen part means "almost," so a penumbra is "almost shadow."

Example Sentences

See Examples For:

penumbra penumbrae penumbras

Wygodny has written original music to subtly accompany Booth’s fable, lending the emotional subtext at points a penumbra of cello.

From Los Angeles Times ● May 29, 2026

A lunar eclipse happens when the Moon passes through the outer region of the Earth's shadow called the penumbra.

From BBC ● Mar. 11, 2025

But the Earth also casts a wider, and lighter, shadow, called the penumbra, which extends away from the planet at an angle.

From Slate ● Apr. 5, 2024

While lit with the penumbra of a spotlight aimed elsewhere, he coolly mimed the smoking of a cigarette with a kazoo.

From New York Times ● Jun. 16, 2023

The late afternoon sun was behind his back, so that he glowed at the edges — shadow, penumbra.

From "Typical American" by Gish Jen

From the top of the terrace he gazed upon this vague form standing up like a phantom in the penumbrae of the evening.

From Salammbo by Gustave Flaubert

Her steps became feebler, and she strained her eyes to look afar upon the naked road, now indistinct amid the penumbrae of night.

From Far from the Madding Crowd by Thomas Hardy

When the ascending currents are powerful, they create those appearances which astronomers designate the nuclei, the penumbrae, the faculae.

From The Story of the Herschels by Anonymous

On a December day in the foothills near Seattle, walking into the sun southward and nearly blinded, all you can see are penumbras of emerald-green moss along the vertical edges of the trunks.

From Seattle Times ● Nov. 18, 2022

Rigging raises a sheet, in front of and behind which the dancers play with shadows, misshapen ones with extra auras and penumbras.

From New York Times ● Sep. 11, 2022

I have been studying the Constitution very carefully, including the emanations of the penumbras, and I can see why people might think there was some inherent right to dinner.

From Washington Post ● Jul. 8, 2022

That opening space sequence functions as supernal ballet, with various vessels spinning, rotating along unfamiliar axes, pirouetting in a post-atmospheric dance of grace against a backdrop of penumbras.

From Slate ● Aug. 6, 2018

The deputy pivoted in the direction of the doorway and shouted at a featureless face broken up in equidistant penumbras by four bars, “Who’s there?”

From "The Great Santini" by Pat Conroy

Related Words

• dark
• gloom
• obscurity
• shade