penumbra

American  

[pi-nuhm-bruh] / pɪˈnʌm brə /

noun

plural

penumbrae, 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.

   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 /

plural

penumbras

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

• penumbral adjective
• penumbrous adjective

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”; 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

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

From BBC • Mar. 14, 2025

The penumbra of Broadway stardust she radiated accumulated over decades.

From Los Angeles Times • Jan. 30, 2024

Supreme Court, fashioned a right to privacy from the penumbra of rights contained in the First, Third, Fourth, Fifth and Ninth Amendments to the U.S.

From Slate • Jan. 18, 2024

His voice is a baritone croon with an electronic penumbra, in a track that hints at old soul translated into ghostly electronics.

From New York Times • Dec. 7, 2022

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

From "Typical American" by Gish Jen

Related Words

• dark
• gloom
• obscurity
• shade