LAKGE SUNSPOTS 



By Seth B, Nicholson 



Mount Wilson Observatory 



Carnegie Institution of Washington 



[With 2 plates] 



Nearly 16,000 groups of sunspots have been recorded since 1874 

 when the Greenwich Observatory began to catalog the spots observed 

 on daily photographs of the sun. These groups range in size from 

 small clusters of tiny spots a few hundred miles in diameter to huge 

 groups nearly 200,000 miles long, containing individual spots as large 

 as 80,000 miles across. Many spots are seen for only a day or two 

 before they disappear, some develop to moderate size, a few grow 

 into groups large enough to be seen without a telescope, and a very few 

 into huge groups like the one which was visible from March 30 to 

 April 13, 191T. 



The area of every group is given for each day in the Greenwich 

 records, the unit of area being one-millionth of a solar hemisphere, 

 or 1,174,000 square miles. Of the 16,000 groups observed since 1874, 

 only 27, less than one-fifth of 1 percent, attained areas as great as 

 2,500 millionths of a solar hemisphere (about 3,000 million square 

 miles). These 27 groups are listed in table 1, with their maximum 

 areas as measured by the Greenwich Observatory or the United States 

 Naval Observatory. The areas are given to only two figures because 

 the irregular outline of a spot seldom permits greater accuracy. 

 Measures by different observers of several of the groups listed differ 

 by as much as 15 percent. Of the spots recorded before 1874, three 

 at least were large enough to have been included in the table. One 

 of these appeared in August 1859, at heliographic latitude 20° N. ; the 

 second in July 1860, at 26° N. ; and the third in August 1860, at 24° S. 

 All the groups listed in table 1 were conspicuous objects with the un- 

 aided eye, if the sun was dimmed sufficiently by fog, smoke, or dark 

 classes. 



1 Reprinted by permission, with revisions (as of September 1947), from Astronomical 

 Society of tlie Pacific Leaflet No. 207, May 1940. 



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