698 THE UNIVERSE. 



The ancient philosophers thought they gave a grand and 

 majestic idea of the sun by comparing its dimensions to the 

 superficies of the Peloponnesus. But what a mean compar- 

 ison ! This torch of the world, this lucerna mundi, as Co- 

 pernicus called it, is of such proportions that if we supposed 

 the earth placed in its centre the mass of the sun would ex- 

 tend beyond the orbit of the moon, and our satellite would 

 only accomplish its revolutions while still buried under the 

 thick incandescent layers of the star which gives us light. 1 



In his " Theogony," Hesiod, wishing to give an idea of 

 the height of the firmament, tells us that an anvil of brass, 



1 The volume of the sun is more than 600 times as large as that of all the plan- 

 ets put together. It turns round its axis in twenty-five days and a half. We may 

 form an idea of the immense bulk of this star relative to that of the earth by 

 means of a comparison mentioned by Arago in his Astronomie Populaire. " A 

 professor of Angers," he says, " hit upon the idea of counting the number of 

 grains of average size contained in the measure of capacity called a litre ; he 

 found there were 10,000. Consequently, a decalitre ought to contain 100,000, a 

 hectolitre 1,000,000, and fourteen decalitres 1,400,000. Having collected the 

 fourteen decalitres of wheat, he showed his audience a single grain, and then 

 said to them, ' This is the size of the earth, while the heap represents the sun.' 

 This comparison occasioned infinitely more surprise among the students than the 

 statement about the relative size of 1 to 1,400,000 in abstract numbers had 

 done." 



If we wish to compare the weight of the sun with that of the earth, astronomy 

 weighs them with as much precision as though each were placed in one of the 

 scales of a balance. The weight of the sun is 2,096,000,000,000,000,000,000,- 

 000,000 tons. That of the earth is only 5,875,000,000,000,000,000,000. 



The physical constitution of the sun has only been made out by the astronomers 

 of our epoch. The body of this star is almost entirely dark, but it is surrounded 

 by three envelopes : one formed of vapors which touch it; another, which is 

 luminous, placed at a great distance, and which is called the photosphere ; and, 

 lastly, a third, which covers the latter, and in which float the clouds. The spots 

 on thesun are occasioned by perforations in the photosphere, which allow us to 

 see the earthy nucleus of the star. See Guillemin, Le del. Paris, 1865. 



