POPULAR SCIENCE 



THE NATURE OF SUN-SPOTS 



By REV. A. L. CORTIE, S.J., F.R.A.S., 



Stonyhurst College Observatory 



A sun-spot consists generally of a large dark area called the 

 umbra, in which at times darker patches can be detected, 

 called nuclei. The umbra is surrounded by a filamentous 

 area of much greater extent, in wavy and broken patches, 

 called the penumbra. Between the umbra and the penumbra 

 is a boundary region of lighter hue, forming the inner edge of 

 the filaments of the penumbra. Photographs on a large scale, 

 showing much detail, as those for instance taken by Father 

 Chevalier at the observatory of Z6-Se in China, confirm the 

 reality of a real increase in light at the interior border of the 

 penumbra of a sun-spot. But they also show that this maximum 

 of luminous intensity does not occur at the extremity of the 

 filaments. These extremities are not in the form of luminous 

 club-heads projecting over the umbra, as figured in Langley's 

 hypothetical drawing of a typical sun-spot, but of tapering 

 points, which curve over towards the umbra, and diminish 

 gradually in brightness. The arrangement of these filaments 

 is in the form of undulating layers, lighter in the regions that 

 are more raised, and darker in those that are at a lower level. 

 A really large sun-spot is of enormous size. For instance, the 

 group, easily visible to the naked eye, which crossed the sun's 

 disc between February 4 and 15, 191 7, in south latitude 15*6° 

 and longitude 10*2°, extended over 18 in longitude and 6*5° 

 in latitude, while its surface area was 26 units, in terms of the 

 1/5000 of the visible disc. Roughly, -this disturbance affected 

 an area of 3,500 millions of square miles, exceeded indeed by 

 the spot of February 7, 1905, which covered 3,800 millions of 

 square miles. 



The life-history of a group of sun-spots generally follows 



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