What name is given to the darkest part of a shadow cast by the earth or moon during an eclipse?

During an eclipse, the shadow cast by a celestial body isn't uniform; it has distinct parts. The very darkest, central part of this shadow is known as the umbra. For an observer located within the umbra, the light source—in this case, the Sun—is completely blocked by the intervening body. If you are standing on Earth as the Moon's umbra passes over you, you will witness the breathtaking spectacle of a total solar eclipse, where day briefly turns into an eerie twilight.

Surrounding this dark core is a much larger but fainter region of shadow called the penumbra. From within the penumbra, the Sun is only partially obscured, resulting in a partial eclipse. The existence of these two distinct shadow regions is why total eclipses are only visible from a very narrow path on Earth's surface, while a partial eclipse can be seen over a much wider area.

The terms themselves are derived directly from Latin. "Umbra" is the Latin word for "shadow." Its counterpart, "penumbra," cleverly combines the Latin "paene," meaning "almost," with "umbra." This literally translates to "almost a shadow," perfectly describing its nature as a zone of partial darkness that fringes the true, total shadow at the center.