Astronomy for the use of schools and academies (1882)
Gillet, J. A. (Joseph Anthony), 1837-1908, Rolfe, W. J. (William James), 1827-1910 · New York : Potter, Ainsworth, & Co. · leaf 198
early as bright as itself,while at the margin it would be seen against one only a ASTRONOMY. 179 quarter as bright. It is evident that the light of any facula,owing to the elevation, would be reduced less rapidly aswe approach the edge of the disk than that of the generalsurface of the sun, which… … lar shape, called the umbra, which is surrounded by aless dark fringe, called the penumbra. The penumbra ismade up, for the most part, of filaments directed radiallyinward. There is great variety in the details of form in differentsun-spots; but they are generally nearly circular during themiddle…
Astronomy
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