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penumbra



  1. In general optics, a penumbra is 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. With particular reference to solar features, a penumbra is 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.

Related category

   • OPTICS AND OPTICAL PHENOMENA