Penumbrae

Penumbrae is the plural form of penumbra, which refers to the partially shaded outer region of a shadow. When an opaque object blocks a light source, it casts a shadow. This shadow is not uniform; rather, it consists of two distinct parts: the umbra and the penumbra. The umbra is the darkest, innermost part of the shadow where the light source is completely blocked. Surrounding the umbra is the penumbra, where the light source is only partially obscured.

In the context of eclipses, penumbrae are particularly relevant. During a solar eclipse, the Moon casts a shadow on the Earth. Observers located within the Moon's penumbra will experience a partial solar eclipse, seeing only a portion of the Sun blocked by the Moon. Similarly, during a lunar eclipse, the Earth casts a shadow on the Moon. If the Moon passes only through the Earth's penumbra, a penumbral lunar eclipse occurs, which is often subtle and difficult to observe as it appears as a slight dimming of the Moon. The size and shape of the penumbra depend on the size of the light source, the size of the object casting the shadow, and the distance between them.