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umbra


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1 The conical inner region of shadow cast by a planet or satellite, from within which the Sun's disk is completely obscured. Passage of a body through this shadow results in its eclipse as, for example, when the Moon traverses Earth's umbra. The dark umbra is surrounded by a broader penumbra (1) in which the Sun's disk is only partially obscured.

2 The central, darkest part of a sunspot. It is also the coolest part, with a temperature of about 4200 K. Umbrae are not uniformly dark but contain light umbral dots. These are smaller versions of photospheric granules, only about 300 km across, and last somewhat longer than granules, up to 25 min. They seem to be penumbral grains that have just entered the umbra.

Subjects: Astronomy and Astrophysics.


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