234 FROM NEBULA TO NEBULA 



There is no regular process for the formation of a spot. 

 Sometimes it is gradual, requiring days or even weeks for its 

 full development, and sometimes a single day suffices. Gener- 

 ally, for some time before the appearance of the spot, there is 

 an evident disturbance of the solar surface, manifested especially 

 by the presence of numerous and brilliant faculae, among which, 

 "pores" or minute black dots are scattered. These enlarge, and 

 between them appear grayish patches, apparently caused by a 

 dark mass lying veiled below a thin layer of luminous filaments. 

 The veil grows gradually thinner, and vanishes, giving us at last 

 the completed spot with its perfect penumbra. The "pores", some 

 of them, coalesce with the principal spot, some disappear, and 

 others constitute the attendant train. When the spot is once com- 

 pletely formed, it assumes usually an approximately circular 

 form, and remains without striking change until its dissolution. 

 As its end approaches, the surrounding photosphere seems to 

 crowd in upon and cover and overwhelm the penumbra. Bridges 

 of light, often many times brighter than the average of the solar 

 surface, push across the umbra, the arrangement of the penumbra 

 filaments becomes confused, and, as Secchi expresses it, the lum- 

 inous matter of the photosphere seems to tumble pell-mell into 

 the chasm, which disappears and leaves a disturbed surface 

 marked with faculae, which in their turn subside after a time. 

 As intimated before, however, the disturbance is not unfrequently 

 renewed at the same point after a few days, and a fresh spot ap- 

 pears just where the old one was overwhelmed. 



The spots usually appear not singly, but in groups at least, 

 isolated spots of any size are less common than groups. Very 

 often a large spot is followed upon the eastern side by a train of 

 smaller ones; many of which, in such a case, are apt to be very 

 imperfect in structure, sometimes showing no umbra at all, often 

 having a penumbra only upon one side, and usually irregular in 

 form. It is noticeable, also, that in such cases, when any con- 

 siderable change of form or structure shows itself in the prin- 

 cipal spot of a group, it seems to rush forward (westward) upon 

 the solar surface, leaving its attendants trailing behind. When 

 a large spot divides into two or more, as often happens, the parts 

 usually seem to repel each other and fly asunder with great ve- 

 locitygreat, that is, if reckoned in miles per hour, though, of 

 course, to a telescopic observer the motion is very slow, since one 

 can only barely see upon the sun's surface a change of place 

 amounting to two hundred miles, even with a very high magnify- 

 ing power. Velocities of three or four hundred miles an hour are 

 usual, and velocities of one thousand miles, and even more, are 

 by no means exceptional. 



The average life of a sun-spot may be taken as two or three 

 months; the longest yet on record is that of a spot observed in 



