n., pl., -brae (-brē), or -bras.
- A partial shadow, as in an eclipse, between regions of complete shadow and complete illumination. See synonyms at shade.
- The grayish outer part of a sunspot.
- An area in which something exists to a lesser or uncertain degree: "The First Amendment has a penumbra where privacy is protected from governmental intrusion" (Joseph A. Califano, Jr.).
- An outlying surrounding region; a periphery: "Downtown Chicago and its penumbra also stand rejuvenated" (John McCormick).
[New Latin pēnumbra : Latin paene, almost + Latin umbra, shadow.]penumbral pe·num'bral or pe·num'brous adj.
(1) The lighter, outer shadow of a double-shadow effect produced by a large, unfocused source of light shining on an interfering opaque object. A penumbral eclipse is one in which the eclipsed object passes through the penumbral shadow. (2) The lighter area, marked by radial filaments, that may surround the darker, central umbra of a sunspot. Penumbral waves are waves observed in the light of H-alpha that move out from the center of a sunspot across the penumbra.
A blurred edge to an image, a halo effect, in an x-ray film caused usually by an overlarge focal spot exacerbated by a long object-to-film distance.
categories related to 'penumbra'
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