The soft, partial shadow that surrounds the complete shadow of an eclipse, or any area of partial illumination between light and dark. It's that mysterious twilight zone where shadows aren't quite shadows and light isn't quite light.
Crafted by 17th-century astronomers from Latin 'paene' (almost) and 'umbra' (shadow). Johannes Kepler needed a word for that puzzling area around a shadow that wasn't completely dark, so scientists essentially invented this term to describe what happens when light bends around objects in space. It moved from astronomy into everyday language to describe any ambiguous, in-between space.
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