Umbra

Umbra, penumbra, and antumbra

Umbra (A) and penumbra (B)

The umbra, penumbra and antumbra are three distinct parts of a shadow, created by any light source after impinging on an opaque object. For a point source only the umbra is cast.

These names are most often used for the shadows cast by celestial bodies, though they are sometimes used to describe levels of darkness, such as in sunspots.

Umbra

Umbra, penumbra and antumbra formed through windows and shutters

The umbra (Latin for "shadow") is the innermost and darkest part of a shadow, where the light source is completely blocked by the occluding body. An observer in the umbra experiences a total eclipse.

Penumbra

The penumbra (from the Latin paene "almost, nearly" and umbra "shadow") is the region in which only a portion of the light source is obscured by the occluding body. 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 within the penumbra.^[1]

In radiation oncology, the penumbra is the space in the periphery of the main target of radiation therapy, and has been defined as the volume receiving between 80% and 20% of isodose.^[2]

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

Antumbra

The antumbra (from Latin ante, 'before') is the region from which the occluding body appears entirely contained within the disc of the light source. An observer in this region experiences an annular eclipse, in which a bright ring is visible around the eclipsing body. If he moves closer to the light source, the apparent size of the occluding body increases until it causes a full umbra.

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