The Sun 217 



as if covered with rice grains, but actually the grains are 

 thousands of miles in diameter and of different shapes. 

 Dark spots, that in reality are dark only by contrast 

 with the more brilliant surface around them, are usually 

 to be seen with a telescope or on photographs of the sun. 

 Sometimes a spot is so large as to be visible if we merely 

 look at the sun through smoked glass. Such a spot is 

 much larger than the earth. The time when the spots 

 are largest and most numerous is called a sun-spot 

 maximum ; when they are least numerous, the minimum. 

 Sun-spot maxima occurred in 1883, 1894, 1906, and 

 1917. Large spots are often seen 2 or 3 years before 

 or after the maximum. The interval, as may be seen 

 from the dates given, is not uniform, but the average 

 since 1610, when the spots were first observed, is 11.13 

 years. This interval is the same as that which inter- 

 venes between the greatest displays of the northern 



FIG. 134. Photograph of a great sun spot taken in February, 1917. The 

 spot is 141,000 miles in length. 



