Definitions

umbra

umbra

[uhm-bruh]
umbra: see eclipse; sunspots.
For other uses of the word "umbra", see Umbra (disambiguation).

For other uses of the word "penumbra", see Penumbra (disambiguation).

The umbra, penumbra and antumbra are the names given to the three distinct parts of a shadow which are created when the source of light is not a point source. These names are usually used when referring to the shadows cast by celestial bodies.

In reality there is no such thing as a point source radiator, though in theory such a light source would cast an umbra, but not an antumbra or penumbra.

Umbra

The umbra (Latin: shadow) is the darkest part of a shadow. From within the umbra, the source of light is completely concealed by the occulting body. In astronomy, an observer in the umbra is said to be experiencing a total eclipse.

Penumbra

The penumbra (Latin: paenes "almost, nearly" + umbra "shadow") 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.

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.

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