File:The elements of astronomy; a textbook (1919) (14595561299).jpg

Identifier: elementsofastro00youn (find matches)
Title: The elements of astronomy; a textbook
Year: 1919 (1910s)
Authors: Young, Charles A. (Charles Augustus), 1834-1908 Young, Anne Sewell, b. 1871., ed
Subjects: Astronomy Constellations
Publisher: Boston, New York (etc.) Ginn and Company
Contributing Library: The Library of Congress
Digitizing Sponsor: The Library of Congress

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Fig. 48. — A Normal Sun Spot. (Secchi; modified.) nucleus gives about one per cent as much light as a correspond-ing area of the photosphere: the blackest portion of a sun spotis really more brilliant than a calcium light. Very few spots are strictly normal. They are often gathered ingroups with a common penumbra, which is partly covered with bril-liant bridges extending across from the outside photosphere.Frequently the umbra is out of the centre of the penumbra, or has apenumbra on one side only, and the penumbral filaments, instead ofconverging regularly towards the nucleus, are often distorted in everyconceivable way. 126 THE SUNS SPOTS. (§186 186. Nature of Sun Spots. — Until very recently sun spotshave been believed to be cavities in the photosphere, filledwith gases and vapors cooler and therefore darker than thesurrounding region. This theory is founded on the fact that
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Fig. 49. — Hydrogen Flocculi around a Sun Spot. many spots as they cross the suns disc look just as if theywere saucer-shaped hollows, with sloping sides colored grayand the bottom black. The penumbra is usually composed of thatch straws, orlong-drawn-out filaments of photospheric cloud, and these, ashas been said, converge in a general way towards the centreof the spot. At its inner edge the penumbra, from the convergence of thesefilaments, is usually brighter than at the outer. The inner ends of thefilaments are ordinarily club-formed, but sometimes they are drawnout into fine points, which seem to curve downward into the umbra,like the rushes over a pool of water. The outer edge of the penum- § 186) DIMENSIONS OF SUN SPOTS. 127 bra is usually pretty sharply bounded, and there the penumbra isdarkest. In the neighborhood of the spot the surrounding photo-sphere is usually much disturbed and elevated into faculae, as shownin Fig. 47. Spectroheliograph pictures (Art. 203) show that

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