350 A Short History of Astronomy CH. xn. 



Herschel obtained (1797) some evidence of variability in 

 the satellites of Jupiter, which appeared to him to support 

 this hypothesis. 



Herschel's observations of other planets were less 

 numerous and important. He rightly rejected the supposed 

 observations by Schroeter (271) of vast mountains on 

 Venus, and was only able to detect some indistinct markings 

 from which the planet's rotation on an axis could be 

 somewhat doubtfully inferred. He frequently observed the 

 familiar bright bands on Jupiter commonly called belts, 

 which he was the first to interpret (1793) as bands of 

 cloud. On Mars he noted the periodic diminution of the 

 white caps on the two poles, and observed how in these 

 and other respects Mars was of all planets the one most 

 like the earth. 



268. Herschel made also a number of careful observa- 

 tions on the sun, and based on them a famous theory of its 

 structure. He confirmed the existence of various features 

 of the solar surface which had been noted by the earlier 

 telescopists such as Galilei, Scheiner, and Hevel, and 

 added to them in some points of detail. Since Galilei's 

 time a good many suggestions as to the nature of spots had 

 been thrown out by various observers, such as that they 

 were clouds, mountain-tops, volcanic products, etc., but 

 none of these had been supported by any serious evidence. 

 Herschel's observations of the appearances of spots suggested 

 to him that they were depressions in the surface of the sun, 

 a view which derived support from occasional observations 

 of a spot when passing over the edge of the sun as a 

 distinct depression or nptch there. Upon this somewhat 

 slender basis of fact he constructed (1795) an elaborate 

 theory of the nature of the sun, which attracted very general 

 notice by its ingenuity and picturesqueness and commanded 

 general assent in the astronomical world for more than half 

 a century. The interior of the sun was supposed to be a 

 cold dark solid body, surrounded by two cloud-layers, of 

 which the outer was the photosphere or ordinary surface of 

 the sun, intensely hot and luminous, and the inner served as 

 a fire-screen to protect the interior. The umbra (chapter vi., 

 124) of a spot was the dark interior seen through an 

 opening in the clouds, and the penumbra corresponded 



