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.