194 M. Arago on the Physical Constitution of the Sun. 



into serious consideration. And I have undertaken to pre- 

 pare the response, not forgetting how uninteresting it may . 

 prove, and that details now become elementary will come 

 prominently into view ; but I trust that your indulgence will 

 not be withheld from him who is fulfilling a duty. 



A general glance at the works of ancient philosophers and 

 modern observers, will prove at once that if the sun has 

 been studied for two thousand years the prospect has often 

 changed, and that during this period the science has made 

 immense advances. 



Anaxagoras maintained that the sun was scarcely larger 

 than the Pelopennesus. 



Eudoxius, who was so much esteemed among the ancients, 

 assigned to the sun a diameter nine times greater than that 

 of the moon. This was a great advance, when compared with 

 the statement of Anaxagoras. But the number given by the 

 philosopher of Gnidus is still immensely short of the truth. 



Cleomidus, who wrote in the reign of Augustus, says that 

 his contemporaries the Epicureans, trusting to appearances, 

 held that the real diameter of the sun did not exceed a foot. 



Let us compare these arbitrary calculations with the con- 

 clusion deducible from the works of modern astronomers, 

 executed with the most minute care, and with the assistance 

 of instruments of extreme delicacy. The diameter of the 

 sun is 883,000 miles ; widely different, as every one will per- 

 ceive, from that stated by the Epicureans. 



Supposing the sun to be spherical, its volume is 1,400,000 

 times that of the earth. Such enormous numbers not being 

 often used in common parlance, and not conveying an exact 

 idea of the magnitude they imply, I shall here employ an 

 illustration which will enable us better to appreciate the 

 immensity of the sun's volume. Imagining, for a moment, 

 that the centre of the sun corresponded to that of the earth, 

 its surface would not only reach the sphere in which the moon 

 revolves, but it would extend almost as far again. 



These results, so extraordinary in their immensity, have 

 the certainty of the elementary principles of geometry on 

 which they are based. 



My subject being so extensive, I shall not in detail insti- 



