The umbra is the darker area of complete shadow, while the penumbra is a lighter area that is only partly shadowed. You can demonstrate this at home. Set up two lamps a couple of feet apart. Hold up...
An umbra is formed when an object blocks all of the light from a light source. A penumbra is formed when light from one source is blocked with light from another source filling in some of the shadow,...
The word "umbra" means "shadow", and the prefix "pen-" means "almost, not complete". So the penumbra is the partial but not total shadow.
The Sun is a million miles in diameter, while the Earth is...