PENUMBRA (Neo-Lat., from Lat. pone, prole, almost + umbra, shadow). When the shadow of an opaque object is thrown upon a surface at some distance from it by a light of considerable apparent size, it is observed that the shadow is divided into two portions, a dark portion in the centre, and a lighter portion sur rounding it. The former is known as the umbra, or complete shadow; the latter as the penumbra, or partial shadow. A reference to the figure will at once make plain their origin and relation; for if S be the illuminating body, E the object whose shadow is cast on the surface, ABCD, it is seen that the small portion, UU' receives (omitting all consideration of refraction, dispersion, etc., of light) no light from S, while the whole surface outside of PPPP' is completely illuminated. The point. P' receives light from the whole of S; the point F is only half illumined, and that by the lower part of S, the illumination of the points becoming less and less as they approach U', which is unillumined. The portion within CU' is the umbra, and that between the boundaries PPPP' and UU' is the penumbra, which, as we have seen, gradually shades from perfect light at the outer boundary to perfect darkness at the inner, so that it is almost impossible exactly to note its limits on either side. This phenomenon, it is
evident, can only occur when the illuminating body is of such a size, real or apparent, as to make the angle, of sensible magnitude; and it is equally evident that the nearer the body E approaches the plane on which its shadow is cast, the larger is the umbra and the smaller the penumbra; while by increasing the distance between E and the plane, so that the point L shall fall between them, the umbra is made to vanish, and the penumbra is increased. This is well illustrated by natural phenomena; the shad ow of a man cast by the sun on the ground presents almost no penumbra : the shadow of the earth thrown by the sun upon space at the dis tance of the moon gives a penumbra many times as large as the umbra; and sometimes. when the moon is new at her apogee, for instance, her shadow cast upon the earth exhibits no umbra. Spectators on the earth who see a partial eclipse of the sun are situated within the penumbra, but within the umbra when they observe a total eclipse: while if the eclipse be annular, the umbra does not exist in the shadow cast by the moon on the earth's surface. See ECLIPSE.