LARGE SUNSPOTS — NICHOLSON 175 



quoted in many books as the area of the largest group on record. 

 Actually that group was a long, narrow stream of spots with an area 

 less than 1,000-millionths of a hemisphere. 



Plate 1 shows the great group of 1947 as it crossed the disk of the 

 sun in March and April. Plate 2 shows four other large spot groups, 

 all to the same scale. A spot group, composed of individual spots of 

 various sizes, is usually elongated in an east-west direction. Each 

 sunspot is composed of an irregular shaded area, sometimes nearly 

 circular, called the penumbra, which is cooler than the sun's surface 

 (the photosphere). Inside the penumbra are smaller, darker areas 

 called umbrae. In large spots the umbrae cover about one-seventh of 

 the area of the spot. Even the umbrae are not black but only less 

 bright than the photosphere ; the contrast between the photosphere and 

 the umbra of a spot is actually less than the reproductions of the photo- 

 graphs would indicate. The temperature of the photosphere is about 

 10,000° Fahrenheit, that of the penumbra about 9,000°, and that of the 

 umbra about 7,600°. Not all umbrae have the same temperature and 

 the largest are not always the darkest and coolest. The radiation from 

 the umbra of a large spot is between one-fourth and one-half that from 

 the photosphere, from the penumbra, between two-thirds and three- 

 fourths. In the large group of 1947, the area of all the umbrae was 

 about 700-millionths of a solar hemisphere, that of the penumbrae 

 about 4,700. As seen projected these areas were, respectively, 0.13 and 

 0.90 of 1 percent of the solar disk. The total solar radiation was there- 

 fore reduced less than one-half of 1 percent by the presence of the 

 large spot group, 



Ever}^ group listed in table 1 except those of October 1894 and Sep- 

 tember 1928, were observed for more than one solar rotation. Groups 

 which attain their maximum area while visible are generally formed 

 on the invisible side of the sun, and those born on the visible side are 

 generally carried out of view before their maximum area is reached. 

 Only 4 of the 27 largest groups were born on the visible half of the 

 sun. Two of these, February 4, 1905, and February 12, 1907, died on 

 the visible hemisphere ; their ages were 91 days and 93 days, respec- 

 tively. The group of February 1946 lasted more than 99 days ; how 

 many more is not known because it developed and disappeared on the 

 invisible hemisphere. Smaller groups have been recorded which lasted 

 longer than any of these. Eleven of the twenty-seven groups in table 1 

 returned only once, nine came back twice, and five returned three times. 



A sunspot is recorded to have been observed for 18 months in 1840-41. 

 The original record of this group has not come to my attention, but it 

 is doubtful whether a spot or a group of spots ever retained its identity 

 for so long a time. Although the same region on the sun may remain 

 active for many months, the continuity of activity is usually due to a 

 succession of spot groups, Sunspots have a habit of reappearing in 



