June 2, 192 1] 



NATURE 



429 



about fifty minutes more than twenty-four hours 

 to make a complete revolution. In other words, 

 all celestial motions take place in the same direc- 

 tion, from east to west. This is a very old idea, 

 several times alluded to by Plato ; but the denial 

 of the independent eastward movement of the 

 planets could not commend itself to any Greek 

 astronomer who realised that the planets moved 

 in orbits considerably inclined to the direction of 

 the daily rotation. This was also the case among 

 the Arabs, and no prominent advocate of a 

 system of spheres appeared among them until the 

 rise of the Aristotelian philo- 

 sophy in Spain in the twelfth 

 century revived the belief in 

 spheres. Al Betrugi (Alpetra- 

 gius) wrote a book on the sub- 

 ject, in which he also let all 

 the motions be from east to 

 west. But though he made 

 some attempt to account for 

 the most conspicuous irregu- 

 larities of the planetary 

 motions, his system is not to 

 be compared with the Ptole- 

 maic system as regards com- 

 pleteness, and it could be 

 accepted only by people who «^ 

 were content merely with the 

 rough outline of a system. 



Early in the thirteenth cen- 

 tury Arabian books on philo- 

 sophy and science began to be 

 known north of the Pyrenees, 

 and along with them came the 

 writings of Aristotle, trans- 

 lated long before from Greek 

 into Arabic, and now from 

 Arabic into Latin. As Aris- 

 totle, who very soon was 

 accepted as an infallible guide, 

 had adopted a system of 

 spheres, one outside the other, 

 it was very difficult for his 

 Christian admirers to do any- 

 thing else. During the whole 

 of the thirteenth century there 

 was a running fight between 

 the adherents of Aristotle 

 (or Alpetragius) and those 

 who realised that no system 

 of concentric spheres could ever account 

 for the observed phenomena so completely 

 as the Ptolemaic system of epicycles and 

 eccentrics did. By the year 1300 the fight 

 was in France fairly well decided in favour of 

 the followers of Ptolemy. But in Italy the study 

 of science had scarcely made any progress ; 

 Ptolemy's great work (though translated into 

 Latin as early as 1175 by Gherardo of Cremona) 

 was quite unknown, and only an extremely 

 elementary text-book by Al Fargani was used in 

 the universities. 



It was therefore natural enough that Dante 

 NO. 2692, VOL. 107] 



should be persuaded of the truth of the doctrine 

 of concentric spheres. Besides, this readily lent 

 itself to poetic treatment, which a complicated set 

 of circles could never do. There is no trace, 

 either in the "Commedia" or in the philosophical 

 treatise the "Convivio," of his having known the 

 " Syntaxis " of Ptolemy. The chief source of his 

 astronomical knowledge is the little text-book of 

 Al Fargani, which he frequently quotes, and from 

 which he occasionally borrows whole passages. 

 In the "Convivio" he repeatedly makes use of 

 the writings of the great scholastic, Albertus 



JfTJisaleiiv, 



Dante's infernal regions. 



Magnus. In contrast with several Italian writers 

 on astronomy even long after his time, who often 

 displayed great ignorance, Dante shows himself 

 well acquainted with the general phenomena of 

 the heavens. Thus he describes correctly the 

 apparent motions of the stars as seen from the 

 poles of the earth or from the equator ; he often 

 indicates the time of year by mentioning the 

 zodiacal sign occupied by the sun ; he even gives 

 a fairly closely correct value of the length of the 

 year. 



In the centre of the universe is the earth, which 

 is a sphere. These two facts were not disputed 



