penumbra

Penumbra is the part of a shadow where light is only partially blocked. It surrounds the darkest region, the umbra, and marks a gradual transition from full shadow to full illumination. The term arises from Latin paene, almost, and umbra, shadow.

In astronomical contexts, penumbrae form because light sources are extended rather than pointlike. The Sun, for example, has an apparent size that produces a surrounding region of partial shadow around the central umbra cast by the Moon or Earth. Observers within the penumbra see a partial reduction in light rather than a total eclipse.

During a solar eclipse, the central area of complete shadow is the umbra, where the Sun is fully blocked. Outside that region lies the penumbra, where only part of the Sun is obscured, producing a partial solar eclipse for observers there. Beyond the penumbra is full illumination. In a lunar eclipse, the Earth’s shadow also includes an umbra and a penumbra; a penumbral lunar eclipse occurs when the Moon passes only through Earth’s penumbra and the shading is subtle.

In optics, the penumbra describes the partially shaded zone around a shadow created by an extended light source. The size and sharpness of the penumbra depend on the relative sizes and distances of the light source and the occluding object.

Overall, penumbra denotes the partial, gradual dimming that occurs at the edge of a shadow rather than complete darkness.