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NATURE 



[June 2, 1921 



The Cosmolog^y of Dante. 

 By Dr. J. L. E. Dreyer. 



THE study of the cosmological ideas set forth 

 in Dante's great poem is of considerable 

 interest, not only because it helps us to under- 

 stand many parts of the poem which otherwise 

 might appear obscure, but also because Dante 

 is a faithful interpreter of the opinions about the 

 construction of the universe which were prevalent 

 in his day. In this respect he is unique among 

 poets. In the present article we shall trace the 

 origin and gradual development of the system of 

 the world adopted by Dante, without entering on 

 minute interpretations of particular passages. 



The principal feature of this system is the 

 arrangement of the universe in a series of con- 

 centric spheres with the earth in the centre (Fig. i). 



Fig. i.^Dante's cosmical system. 



This is a very old idea, originating in the most strik- 

 ing of all celestial phenomena, the rotation of the 

 heavens in twenty-four hours from east to west. 

 Eudoxus (about 370 b.c.) was the first to design 

 a complete system of concentric spheres, arranged 

 so as to account for this rotation of the heavens 

 as well as for the principal irregularities in the 

 motions of sun, moon, and planets in the opposite 

 direction. Though Eudoxus was a great mathe- 

 matician, his system of spheres could account only 

 partially for the observed phenomena, probably 

 because it was founded on an utterly insufficient 

 number of observations of these phenomena. The 

 system was much improved by Kalippus, and, 

 what was of more importance, it was accepted by 

 Aristotle. To him the spheres were not merely 

 mathematical conceptions, but physically existing 



NO. 2692, VOL. 107] 



bodies, kept in motion by the soul of each. Not- 

 withstanding his great authority, however, this 

 system was rejected by the astronomers of Alex- 

 andria, chiefly because it suffered from an incur- 

 able defect, that of assuming every planet to be 

 always at the same distance from the earth. That 

 this is not the case is clearly shown by the variable 

 brightness of the planets, most strikingly seen in 

 the cas6 of Venus and Mars. 



The increased number of apparent irregularities 

 in the planetary motions revealed by steadily 

 pursued observations led to the development of a 

 system of the world which finally became known 

 as the Ptolemaic system, because the last touches 

 were put to it by Claudius Ptolemy in the second 

 century a.d. It assumed a planet to move on 

 a circle, the epicycle, the centre of which 

 moved on a larger circle, the eccentric, near 

 ^he centre of which the earth was situated. 

 Additional refinements were added to account 

 for observed minor irregularities. But all 

 this, though very satisfactory to mathemati- 

 cians, was not to the taste of many people, 

 who could not accept all these circles as 

 realities, but demanded some sort of a system 

 of spheres, not necessarily concentric. To 

 satisfy this demand it was suggested that 

 we might for the epicycle substitute a small 

 sphere, to the surface of which a planet was 

 attached, while the sphere fitted in and 

 moved between the surfaces of two concen- 

 tric spheres, near the common centre of which 

 the earth was placed. Ptolemy, who wrote 

 a valuable text-book on astronomy (the 

 " Syntaxis," generally known by the Arabic 

 name " Almagest "), wrote also, for the 

 weaker brethren, another, called " The 

 Second Book of Phenomena," in which a 

 complicated system of spheres is described. 

 But this was never a success, and the Greek 

 original is lost, so that the Ijook was quite 

 unknown to modern European readers until 

 1907, when a German translation from an 

 Arabic version was at last published. 

 Among the Arabs we find an attempt to adopt 

 material spheres in the cosmical system of the 

 "Brethren of Purity," a semi-religious society 

 which arose at Basra near the end of the tenth 

 century. They taught that there are nine spheres 

 of different thicknesses, fitting inside each other 

 "like the skins of an onion." The ninth sphere 

 is the prime mover, and turns in twenty-four 

 hours. The eighth sphere is that to which the 

 fixed stars are attached; it revolves in a very 

 slightly longer period, lagging behind to the 

 amount of one degree in a hundred years. This 

 is supposed to account for the precession of the 

 equinoxes. Saturn (the seventh sphere) lags more, 

 and the motion becomes slower as we descend 

 through the spheres, until we reach the first or 

 slowest sphere, that of the moon^ which takes 



