Umbra, penumbra and antumbra.

Example of umbra, penumbra and antumbra outside astronomy.

The umbra, penumbra and antumbra are the names given to three distinct parts of a shadow, created by any light source. For a point source only the umbra is cast.

These names are usually used when referring to the shadows cast by celestial bodies.

[edit] Umbra

The umbra (Latin for "shadow") is the darkest part of the shadow. From within the umbra, the source . In astronomy, an observer in the umbra is said to be in the shadows experiencing total eclipse.

[edit] Penumbra

The penumbra (from the Latin paenes "almost, nearly" and umbra "shadow") is the region in which only a portion of the occulting body is obscuring the light source. An observer in the penumbra experiences a partial eclipse.

An alternative definition is that the penumbra is the region where some or all of the light source is obscured (i.e., the umbra is a subset of the penumbra). For example, NASA's Navigation and Ancillary Information Facility defines that a body in the umbra is also in the penumbra.^[1]

[edit] Antumbra

The antumbra is the region from which the occulting body appears entirely contained within the disc of the light source. If an observer in the antumbra moves closer to the light source, the apparent size of the occulting body increases until it causes a full umbra. An observer in this region experiences an annular eclipse.

Earth's shadow, to scale, showing the extent of the umbral cone beyond the Moon's orbit (yellow dot, also to scale).

[edit] References