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The penumbra ( Latin for mid- shadow) is the portion of a shadow that results from the source of illumination being only partially blocked. Penumbras only occur when the source of light is not a point-source. As the sun is a visible disc, solar shadows have penumbras. The penumbra part of a shadow is contrasted with the umbra part of the shadow in which the light source is completely blocked.Also, the penumbra is the lighter, fuzzier outer edge of a shadow.
The shadow is darkest at the umbra, near the object, and weakest at the penumbra, at the edge.
The penumbra is also the ring-shaped semi-bright region of a sunspot that surrounds the darker umbra.
See also: eclipse.
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