The call of the stars; a popular introduction to a knowledge of the starry skies with their romance and legend . es, however, they dis-appear in a day, and again they have been known tolast as long as eighteen months. They are, as demon-strated by Hale, in 1908, probably great electricalvortices or whirling storms, and are regarded as some-what analogous to cyclones, tornadoes, or water-spoutson the earth. The vortex in the solar atmosphere has,it is believed, a spiral structure, and may be eitherright-handed or left-handed in its direction, while theeffect of the vortical motion is to draw in
The call of the stars; a popular introduction to a knowledge of the starry skies with their romance and legend . es, however, they dis-appear in a day, and again they have been known tolast as long as eighteen months. They are, as demon-strated by Hale, in 1908, probably great electricalvortices or whirling storms, and are regarded as some-what analogous to cyclones, tornadoes, or water-spoutson the earth. The vortex in the solar atmosphere has,it is believed, a spiral structure, and may be eitherright-handed or left-handed in its direction, while theeffect of the vortical motion is to draw into the spotgases from the upper solar atmosphere, which areejected from the spot at lower levels. The gasespresent in the nucleus of a sun-spot have a consider-ably lower temperature than on the surface of the Sunoutside of the spot, owing, it is thought, to their coolingby expansion in the axis of the vortex. In a typical spot, the dark central portion is calledthe umbra, and the lighter irregularly shaded greyportion around it the penumbra. The umbra of aspot is not really dark but only relatively so with. Yerkes Observatory Plate XXI, The Great Sun-Spot of July 17, 1905 The Sun 289 respect to the brilliant envelope of the Sun—the photo-sphere—as it is actually more brilliant than the electricarc. Frequently a spot will be seen to be crossed byone or more brightly shining plume-like projectionscalled bridges (Plate XXL). Generally the surfacein the neighbourhood of the sun-spots seems more orless raised, and the spots themselves are usually abovethe general surface of the photosphere. In size, sun-spots vary greatly, ranging from five hundred to fiftythousand miles in diameter, and sometimes a penumbrasurrounding a group of spots will measure from one totwo hundred thousand miles across. The area of onespot photographed at Greenwich in March, 1905, wasnearly forty times that of the entire surface of the infrequently sun-spots are large enough to beseen with the
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