umbra


umbra
umbral, adj.
/um"breuh/, n., pl. umbras, umbrae /-bree/.
1. shade; shadow.
2. the invariable or characteristic accompaniment or companion of a person or thing.
3. Astron.
a. the complete or perfect shadow of an opaque body, as a planet, where the direct light from the source of illumination is completely cut off. Cf. penumbra (def. 1a).
b. the dark central portion of a sunspot. Cf. penumbra (def. 1b).
4. a phantom or shadowy apparition, as of someone or something not physically present; ghost; spectral image.
[1590-1600; < L: shade, shadow]

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      that part of a shadow in which all light from a given source is excluded. The shadow from a point source of illumination is essentially all umbra, but that from a source of some size (as from the Sun) consists of a small umbra and a much larger partial shadow called the penumbra. Thus, in an eclipse of the Sun, the regions within the umbra experience a total eclipse and those within the penumbra, partial eclipse. The term is also used for the dark central portion of a sunspot.

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Universalium. 2010.

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