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”; 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

Boston Scientific on Thursday said the deal values Penumbra at $374 a share, a 19% premium to Wednesday’s closing price of $313.43 for the Alameda, Calif., company.

From The Wall Street Journal • Jan. 15, 2026

The addition of Penumbra, which makes mechanical thrombectomy products for vascular procedures, expands Boston Scientific’s vascular medicine business.

From Barron's • Jan. 15, 2026

Boston Scientific is spending $14.5 billion in cash and stock to buy Penumbra, another medical-device maker that makes products used in minimally invasive procedures to treat conditions such as pulmonary embolism and stroke.

From MarketWatch • Jan. 15, 2026

Park includes references to “Same Bed Different Dreams”: Penumbra College, for instance, appears in both books.

From Los Angeles Times • Jul. 24, 2025

Penumbra, pē-num′bra, n. a partial or lighter shadow round the perfect or darker shadow of an eclipse: the part of a picture where the light and shade blend into each other.—adjs.

From Chambers's Twentieth Century Dictionary (part 3 of 4: N-R) by Various

Related Words

• dark
• gloom
• obscurity
• shade