What is the penumbra?

The penumbra is described as the area of partial shadow that occurs during an eclipse, rather than a complete shadow. It is the part that is lighter, but still not fully illuminated. This allows for a gradient effect where the light is partially blocked but still shines through to some extent. In terms of an eclipse, the penumbra will indeed be the larger region surrounding the umbra (the complete shadow), where observers see a partial blockage of the sun.

In contrast, the other descriptions do not accurately reflect the nature of the penumbra. The center of a shadow refers to the umbra, which is different from the penumbra. The brightest part of sunlight describes direct sunlight and does not pertain to shadow regions. An area with no shadow typically indicates full illumination, which does not correlate with the concept of the penumbra where shadows are present, albeit partially.