umbra

Umbra is the darkest part of a shadow, the region where an opaque object completely blocks all direct light from a light source. The word comes from Latin umbra, meaning shade or shadow. In astronomical contexts, shadows are typically conical because the light source has finite size; the umbra narrows with distance and may end at a point or fail to reach a target.

During a solar eclipse, the Moon passes between the Sun and Earth. If an observer lies within the Moon’s umbra, the Sun is entirely hidden and a total solar eclipse is seen. If the Moon is farther away or the alignment is such that the Sun is only partially obscured, observers are in the antumbra, resulting in an annular eclipse, while the penumbra produces a partial eclipse.

In a lunar eclipse, the Earth casts its shadow on the Moon. When the Moon passes through the Earth’s umbra, a total lunar eclipse occurs; partial eclipses occur when only part of the Moon enters the umbra, and a penumbral lunar eclipse occurs when the Moon traverses only the penumbra, producing a subtler dimming.

Beyond eclipses, umbra describes the darkest part of any shadow cast by an opaque body. The related terms penumbra (partial shadow) and antumbra (region beyond the umbra where the light source remains partly visible) complete the framework for describing shadows and eclipse phenomena. The exact extent of an umbra depends on the relative sizes and distances of the light source and occluding object.