SUNSPOTS AND SOME OF THEIR PECULIARITIES. 



61 



which have been cleai'ly recognised, such as, for instance, titanium 

 oxide, magnesium hydride and water vapour, and the absorption 

 due to these accounts for much of the apparent darkness of the 

 spot. Round the spot the photosphere is noticeably brighter, 

 and in cases of great activity, very brilliant " bridges " flow 

 into the penumbra, and cross the umbra, and the spectroscope 

 shows many of the dark lines strongly reversed over or near the 

 spot ; all symptoms of increased temperature. 



The peculiarity of sunspots which has inevitably attracted the 

 greatest attention, is the way in which the sun's activity waxes 

 and wanes. Thus, in the year 1913, the sun's disc was free 

 from spots on no fewer than 312 days, and the average daily 

 spotted area for the entire sun was only 8 millions of square miles. 

 In 1917, on the other hand, there were no days upon which no 

 spots were seen, and in August of that year the average daily 

 area under sunspots exceeded 2500 millions of square miles. 



This immense change in the extent to which the solar surface 

 is disturbed does not take place capriciously, but appears to 

 follow a progression with many features of regularity. The 

 interval of time from one quiet period to the next, or from one 

 very active period to the next, generally exceeds ten years, and 

 is less than twelve ; on the average, the Sunspot Cycle, as it is 

 called, is 11* 1 years in length. 



When Galileo first noticed sunspots, he was not content with 

 observing their appearance and form, and the changes which 

 they passed through, but he measured their apparent positions 

 on the sun's disc from day to day, and in this way proved that 

 the spots were not dark bodies floating between the earth and 

 the sun, but were actually upon the sun's surface. From this he 

 learned that the sun rotated on an axis, the position of which he 

 soon determined. Then he learned that spots were not equally 

 distributed over the sun's surface, but were found in zones, 

 north and south of the equator ; very few, and those but small, 

 being detected at a distance from the equator of more than 40°. 



One of the most striking peculiarities of sunspots is that their 

 distribution in latitude varies with the progress of the sunspot 

 cycle. If we were to commence observations of the sun at a time 

 when the solar activity was far advanced in decline, we should 

 find that the spots were practically confined to the zone lying 

 between north latitude 10° and south latitude 10°. Very soon, 

 however, spots would begin to appear at about 30° from the 

 equator, both north and south, and for a time three sunspot 



