174 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1947 



Table 1. — TTie largest suns-pot groups 



Date 



1892, 

 1893, 

 1894, 

 1896, 

 1897, 

 1905, 

 1905, 

 1905, 

 1907, 

 1907, 

 1917, 

 1917, 

 1920, 

 1925, 



Feb. 12.. 

 Aug. 7.- 

 Oct. 8.-- 

 Sept. 17. 

 Jan. 9... 

 Feb. 4... 

 Mar. 8.. 

 Oct. 20.. 

 Feb. 12.. 

 June 20.. 

 Feb. 10.. 

 Aug. 10. 

 Mar. 22. 

 Dec. 29.. 



Date 



1926, 

 1928, 

 1937, 

 1937, 

 1937, 

 1938, 

 19.38, 

 1938, 

 1939, 

 1939, 

 1946, 

 1946, 

 1947, 

 1947, 



Jan. 24.. 

 Sept. 27- 

 Jan.31-- 

 July29_. 

 Oct. 5... 

 Jan. IS.. 

 July 15- . 

 Oct. 12.- 

 Sept. l_. 

 Sept. 10. 

 Feb. 5... 

 July 26.. 

 Mar. 10. 

 Apr. 7... 



* Milllonths of a solar hemisphere. To express the area in million square miles increase these figures 

 by one-sixth. 



' The large group in April 1947 was probably identical with the one in March, both being in the same 

 region on the sun. They have been considered as one group in this article. 



The great spot group of April 1947, which was the largest ever 

 recorded, developed from some small spots first seen on February 5, 

 1947, at the east limb of the sun in latitude 23° S. These spots grew 

 rapidly until by February 7 they formed a group large enough to be 

 seen without a telescope. When, after a solar rotation, the group 

 reappeared at the east limb on March 3, it had changed considerably, 

 being much larger and more compact than in February. In March 

 the group was composed primarily of one huge spot, which had appar- 

 ently developed from the preceding (western) members of the Febru- 

 ary group, the following members of which had disappeared. When 

 it reappeared on March 30 for its third transit, the huge group al- 

 though brealring up had increased in area. When largest it covered 

 more than 1 percent of the solar disk, its area being 6,300,000,000 

 square miles, 5,400-millionths of a solar hemisphere. On its fourth 

 and last return the group was much smaller. The preceding part, 

 which had always been the smaller, disappeared on May 7. The fol- 

 lowing part passed around the west limb on May 11 and did not return. 



The large spot of March 1947 was the largest single spot on record 

 with an area of 5,000,000,000 square miles, 4,300-millionths of a solar 

 hemisphere. Before 1874, visual observers sometimes published the 

 over-all length and breadth of a group instead of its area. From this 

 data the area cannot be computed because the group may have been 

 composed of several unconnected spots. A long spot group observed 

 in September and October 1858 has been cited in several popular books 

 on astronomy as the largest group ever recorded. H. Schwabe, a 

 noted observer of sunspots, gave its east-west diameter as 321'.'3, 

 which is equivalent to 143,000 miles or one-sixth of a solar diameter. 

 Someone, interpreting this figure as the diameter of a huge circular 

 spot, computed its area as one thirty-sixth that of the solar disk, or 

 14,000-millionths of a solar hemisphere. This figure has since been 



