Penumbra << pih NUHM bruh >> means partial shadow. Suppose an object such as the earth blocks the light given off by a larger source such as the sun. Behind the earth, there is a dark shadow where none of the sun’s rays pass. Outside this dark region is another, less dark region, where some of the light passes. This partially dark region is called the penumbra. The inner dark region is called the umbra. The moon is almost totally dark when it passes through the umbra of the earth’s shadow. This occurrence is called a total eclipse of the moon.
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You can see an umbra and a penumbra in the shadow of your hand. Hold your hand a few feet from a lamp in a room with no other light. Place a piece of white paper on the other side of your hand. Your hand casts a shadow on the paper. In the center of the shadow of each finger is the dark umbra. Around each umbra is a brighter penumbra.
