During a lunar eclipse, the penumbra refers to the outer part of the Earth’s shadow. A lunar eclipse occurs when the Earth comes directly between the Sun and the Moon, causing the Earth to cast a shadow on the Moon.
The Earth's shadow consists of three parts:
- Umbra: The central, darkest part of the shadow where the Earth blocks all direct sunlight from reaching the Moon.
- Penumbra: The lighter outer part of the shadow where the sunlight is only partially blocked. In this region, some sunlight still reaches the Moon, causing a subtle shading effect.
- Antumbra: This part of the shadow can occur in solar eclipses, but it's not relevant for lunar eclipses. It’s the region where the light from the Sun is completely obscured by the Earth but allows a ring of sunlight to be visible around the Earth's silhouette.
During a penumbral lunar eclipse, the Moon passes through the penumbral shadow of the Earth, leading to a subtle shading on the Moon's surface without the dramatic darkening seen during a total lunar eclipse. Although penumbral eclipses are not as easily noticeable, they are one type of lunar eclipse that occurs when the Moon does not enter the umbra.