penumbra
1. a fringe region of half shadow resulting from the partial obstruction of light by an opaque object
2. Astronomy the lighter and outer region of a sunspot
3. Painting the point or area in which light and shade blend
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005
penumbra
(pi-num -bră) 1. See umbra.
2. See sunspots.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006
penumbra
[pə′nəm·brə] (astronomy)
The outer, relatively light part of a sunspot.
(optics)
That portion of a shadow illuminated by only part of a radiating source.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.
Penumbra
the space between regions of total shadow and total light formed behind an opaque body illuminated by a light source whose dimensions are comparable both with the dimensions
Figure 1
of the body and with the distance from the light source to the body (see Figure 1). Only part of the light source is visible in the penumbra; the light source is not visible in the umbra and is completely visible in the region of total light. The point of observation of a partial solar eclipse is within the penumbra formed by the moon as illuminated by the sun.
The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.