An umbra is the darkest, most complete part of a shadow, where light is totally blocked. When you hold your hand between a lamp and the wall, you'll see a dark shadow in the middle with fuzzy edges around it. That dark center is the umbra.

Scientists use it most often when describing eclipses. During a solar eclipse, the Moon's umbra is the shadow's core that falls on Earth. If you stand in that zone, you see the Sun completely covered. Outside the umbra, in the lighter shadow called the penumbra, you'd see only a partial eclipse.

You can create your own umbra by holding a ball between a bright light and a surface. The sharp, dark shadow directly behind the ball is the umbra, while the fuzzier, lighter shadow around it is the penumbra. The umbra exists because the ball completely blocks the light from reaching that spot.

Astronomers also use umbra to describe the darkest part of a sunspot, those cooler regions that appear as dark patches on the Sun's surface.