What is penumbra?

A penumbra is like the soft shadow you see when part of a light source is blocked, not all of it.

Imagine you're sitting in your favorite sunny spot, and there’s a big, round ball between you and the sun. That ball isn’t completely blocking the sun, just part of it. So instead of being totally dark, you see a soft shadow around you. That soft shadow is the penumbra.

Think of the sun as a giant cookie, and the ball in between as someone taking a bite out of it. If only part of the cookie is eaten, some light still comes through, that’s why your shadow isn’t completely dark. The penumbra is like the edge of the shadow from that half-eaten cookie.

You might see a penumbra during an eclipse, when the moon only partly covers the sun. Or maybe you've noticed it when you're sitting in a room with a lamp and someone walks between you and the light, your shadow gets softer around the edges, and that’s the penumbra doing its job!