Umbra

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For other uses of the word "umbra", see Umbra (disambiguation).

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

Umbra and penumbra

Umbra, penumbra, and antumbra

The umbra (Latin: "shadow") is the darkest part of a shadow. From within the umbra, the source of light is completely blocked by the object causing the shadow. This contrasts with the penumbra, where the light source is only partially blocked and there is only a partial shadow.

Penumbras occur only when the source of light is not a point source. As the sun is a visible disc, solar shadows have penumbras.

The part of the penumbra where an annular eclipse is visible is called the antumbra. In an annular eclipse, the moon is not of sufficient size to completely cover the sun, and so its shadow is not long enough to reach the Earth and result in a total solar eclipse. At maximum eclipse, the moon is surrounded by a ring, or annulus, of light, and the location on Earth where the ring can be seen is also the location of the antumbra. Although the antumbra may be referred to as a "negative shadow", it is nevertheless darker than the penumbra, just like the umbra is in a total eclipse. However, whereas the umbra is completely devoid of sunlight, the antumbra is not because of the remaining ring.

[edit] Related usages

The umbra is also the comparatively dark central region of a sunspot.
The penumbra of a radiation field is the region, at the edge of the radiation beam, over which the dose rate changes rapidly as a function of distance from the beam axis. In radiation medical fields, it is also defined as "The edge region surrounding a beam where the energy level drops from 80% to 20% of its maximum."
An area in which something exists to a lesser or uncertain degree is sometimes called a penumbra: "The first nine amendments to the Bill of Rights have a penumbra where privacy is protected from governmental intrusion.” - Griswold v. Connecticut, U.S. Supreme Court
The ischemic penumbra is the zone around an ischemic brain infarction which receives some degree of collateral circulation.