antumbra

Antumbra is a term used in the study of eclipses to describe the portion of a shadow in which the occulting body is smaller than the light source, so the occluded object does not completely cover the source. The word comes from Latin anti- "opposite" and umbra, shadow. It is most commonly applied to solar eclipses.

In solar eclipses, the antumbra lies beyond the Moon's umbra. It occurs when the Moon is at or near its apogee, so its apparent diameter is smaller than that of the Sun. Observers located within the antumbra see an annular eclipse, in which the Sun's disk remains visible around the silhouette of the Moon as a bright ring. The border between the antumbra and the Sun's outer disk marks the annular contact points; as the Moon moves, the ring may appear to shrink or expand.

An annular eclipse path on Earth is narrower than a totality path and lasts only a few minutes at any given site; the exact duration depends on geometry such as distance and relative motion. Outside the antumbra or umbra, the Sun is only partially eclipsed or not eclipsed at all.

Antumbra contrasts with umbra and penumbra, which describe the regions of full shadow and partial shadow respectively. In lunar eclipses, the terms umbra and penumbra are used for Earth's shadow on the Moon; the antumbra is generally not involved.