M. Arago on the Physical Constitution of the Sun. 195 



tute a comparison between the results — truly absurd from 

 their insignificancy — at which the ancients had arrived regard- 

 ing the distance between the sun and the earth, and those which 

 modern observations have deduced. I shall even limit my- 

 self to remark that it is demonstrated — and I use this positive 

 term advisedly — that it is demonstrated, since the transit of 

 Venus in 1769, that the mean distance from the sun to the 

 earth is 95 millions of miles, and that between summer 

 and winter the sun removes itself from us more than three 

 million of miles. Such is the distance of the immense 

 globe whose physical constitution modern astronomers have 

 succeeded in determining. — We find nothing in the works of 

 ancient philosophers on this subject, which merits a moment's 

 consideration. 



Their disputes as to whether the sun is a pure or impure, 

 an extinguishable or unextinguishable fire, not being sup- 

 ported by any observation, left in profound obscurity the pro- 

 blem which the moderns have tried to solve. 



The progress which has been achieved in this inquiry, dates 

 from 1611. At this epoch, little removed from that of the 

 invention of the telescope, Fabricius, a Dutch astronomer, 

 saw black spots distinctly exhibited on the eastern margin 

 of the sun, which moved gradually towards the centre, passed 

 it, reached the w^estern margin, and then disappeared for a 

 certain number of days. 



From these observations, frequently since repeated, this 

 conclusion may be deduced, that the sun is a spherical body, 

 endowed with a rotatory motion, whose duration is equal to 

 twenty-five and a half days. 



These black spots are irregular and variable, but well- 

 defined towards their circumference ; they are sometimes of 

 considerable dimensions, some having been seen five times 

 the size of the earth ; they are generally surrounded with a 

 radiance perceptibly less luminous than the rest of the orb» 

 and which has been named penumbra. This penumbra, first 

 noticed by Galileo, and carefully observed by his astrono- 

 mical successors in all the changes which it undergoes, has 

 led to a supposition, concerning the physical constitution of 

 the sun, which at first must appear altogether astonishing. 



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