Here's why:

* Umbra: The umbra is the darkest part of a shadow. It's the region where the light source (the Sun) is completely blocked by the object casting the shadow (Earth).

* Penumbra: The penumbra is the lighter, outer part of a shadow. Here, the light source is partially blocked.

* Lunar Eclipse: During a lunar eclipse, the Earth passes directly between the Sun and the Moon. This means the Moon enters Earth's shadow.

Therefore, during a total lunar eclipse, the Moon is completely immersed in Earth's umbra, which is why the Moon appears a reddish color.