THE SUN. 



73 



sun upon the axis is in tha same direction as the motion of the planets round 

 the sun, that is to say, from west to east. 



SPOTS ON THE SUN. 



One of the earliest fruits of the invention of the telescope was the discovery 

 of the spots upon the sun, and the examination of these has gradually led to a 

 knowledge of the physical constitution of the centre of our system. 



When we submit a solar spot to telescopical examination, we discover its 

 appearance to be that of an intensely black irregularly-shaped patch, edged with 

 a penumbral fringe, the brightness of the general surface of the sun gradually 

 fading away into the blackness of the spot. When a spot is watched for a con- 

 siderable time, it is found to undergo a gradual change in its form and magni- 

 tude ; at first increasing gradually in size, until it attains some definite limit of 

 magnitude, when it ceases to increase, and soon begins, on the contrary, to 

 diminish ; and its diminution goes on gradually, until at length the bright sides 

 closing in upon the dark patch, it dwindles first to a mere point, and finally 

 disappears altogether. The period which elapses between the formation of 

 the spot, its gradual enlargement, subsequent diminution, and final disap- 

 pearance, is very various. Some spots appear and disappear very rapidly, 

 while others have lasted for weeks and even for months. The magnitudes 

 of the spots are in proportion to the magnitude of the sun itself. At the 

 distance of the sun, a spot, the magnitude of which would be barely visible, 

 must have a diameter of four hundred and sixty miles, and an area of one 

 hundred and sixty-six thousand square miles, which is, therefore, the smallest 

 space on the surface of the sun which would be distinctly seen. Among the 

 many spots which have been recorded, one was observed by 'Mayer, the 

 area of which was about fifteen hundred millions of miles square, or about 

 thirty times the surface of the earth. 



Spots have been occasionally seen on all parts of the sun, but that region on 

 which they are found generally to prevail, is one which corresponds with the 

 tropical parts of the earth, that is, a space extending about thirty degrees on 

 either side of the solar equator. 





