- The partially shaded outer region of the shadow cast by an opaque object.
- The shadow cast by the Earth or moon over an area experiencing a partial eclipse.
“The moon cast a hazy penumbra over a section of Earth during the partial eclipse.”
“The sun was going down, but I could still see enough to walk by following the penumbras of the shadows.”
“I kept a close eye on the penumbra of the shadow so I would know the second the door started to move.”
Latin, mid-17th century
Why this word?
Eclipse watchers should know this word and its counterpart, “umbra.” During a partial eclipse, two shadows are cast. The umbra is the darker center that gets smaller as it moves away from the sun, and the penumbra is the smaller, lighter shadow that gets larger as it moves away from the sun. “Penumbra” comes from the Latin roots of “paene-” (meaning “almost”) and “umbra” (meaning “shadow”). Even when there’s not an eclipse, take a look around for the “almost shadow” around the edges of sharper shadows — as the angle of light changes, so will the penumbra.
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