NA TURE 



359 



THURSDAY, DECEMBER 3, 191 4. 



DANTE'S ASTRONOMY. 

 Dante and the Early Astronomers. By M. A. Orr 

 (Mrs. John Evershed). Pp. xv + 507. (London : 

 Gall and Inglis, n.d.) Price 155. net. 



DANTE'S cosmological ideas and his exposi- 

 tion of medieval astronomical knowledge 

 have been discussed by several of the numerous 

 commentators of his writTngs. It is, however, 

 useful to have them collected in one place and 

 discussed by a writer having a sound knowledge 

 of modern astronomy. Mrs. Evershed has there- 

 fore done good and valuable work by producing 

 the present volume. After a rapid sketch of the 

 principal celestial phenomena visible to the naked 

 eye without instruments, she gives a popular 

 account of the progress of Greek astronomy. The 

 "list of principal authorities" at the beginning 

 of the book shows that this account is mainly 

 founded on some of the principal modern books 

 and monographs on this subject. There are very 

 few ancient writers in the list, e.g. Hipparchus, 

 Ptolemy, Seneca, and others are conspicuous by 

 their absence. On the other hand, Cicero, "De 

 Senectute " is included, though there is not a word 

 about astronomy in this little book except an 

 allusion to Gallus, who was fond of predicting 

 eclipses. 



On the whole, the account of Greek astronomy 

 is correct, but it would have been a help to 

 readers who may desire further information if 

 references to the sources employed had been given 

 in each chapter; for instance, on p. 299 (note) the 

 authority is no doubt Smyth's "Cycle of Celestial 

 Objects," vol. ii., p. 332, which should have been 

 quoted. Want of knowledge of the original 

 writers occasionally leads the author astray, as, 

 for instance, when she gives a picture of the 

 great equatorial at Peking, and seems to think 

 that this was a copy of one of Ptolemy's instru- 

 ments. There is no mention of equatorials in the 

 Almagest, and it is not even certain that the 

 Arabs constructed them, though it is probable 

 enough, since the idea of the Peking instrument 

 appears to be due to a Persian, Gemal-ed-din. On 

 p. 122 it is said that Hipparchus gives a list of 

 sixteen time-stars culminating at intervals of an 

 hour exactly, and that his knowledge of spherical 

 trigonometry enabled him to calculate the time of 

 night to within one minute. But there were forty- 

 four of these stars, and their use did not involve 

 any calculation, as they only served to regulate 

 the water-clocks. 



Passing to the Middle Ages, the importance of 

 astrology in those days is duly pointed out. But 

 NO. 2353, VOL. 94] 



it is not correct to call the art of predicting the 

 fate of an individual "judicial astronomy," as the 

 author persists in doing. It was called "judicial 

 astrology," not to distinguish it from the science 

 of stellar motions, but to show that it was a 

 special branch of the more exalted, philosophical 

 astrology, which sprang from the loftiest views of 

 Greek thinkers about the unity of the Kosmos and 

 the inter-dependence of all parts thereof. The 

 astronomical work done by the Arabs is briefly 

 mentioned, and it is justly emphasised that Dante 

 owed his knowledge of Alexandrian astronomy 

 solely to the text-book of Alfraganus (Al Fargani), 

 which he had thoroughly mastered, so that we 

 have in the "Divine Comedy" and the "Convito" 

 an excellent picture of the state of science at the 

 beginning of the fourteenth centurj-. It is only 

 very rarely that Dante gives any astronomical 

 information not to be found in Alfraganus, one of 

 the principal cases being the statement that the 

 length of the tropical year is i/iooth of a day 

 less than 365J days. The author says it would be 

 interesting to know where Dante found this value. 

 We think there can be little doubt that he got it 

 from the " Computus Maior " of Johannes Cam- 

 panus, a Canon of Paris, in the thirteenth century, 

 which is extremely likely to have been know^n to 

 Dante's teacher, Brunetto Latini, who lived some 

 vears in France, though he does not mention the 

 small amount of i/iooth day in his "Tresor," 

 where he gives only round numbers. 



After giving a classified list of the allusions to 

 sun, moon, planets, and stars in the " Divine 

 Comedy," the author discusses the time-indications 

 given through references to the positions of the 

 heavenly bodies in the course of the poet's 

 journey. They show that Hell, a conical cavity 

 reaching to the earth's centre, was entered in the 

 evening and left twenty-four hours later, while 

 the ascent to the other side of the earth took 

 nearlv as long. Purgatory is a conical mount on 

 an island in the midst of the watery hemisphere at 

 the antipodes of Jerusalem. The time spent on it 

 brings us to the morning of the seventh day, and 

 the ascent through the seven spheres of the 

 planets, the sphere of the Fixed Stars, and that 

 of the Primum Mobile to the Empyrean took 

 eighteen hours. As regards the assumed date of 

 the journey, it has generally been supposed that 

 this was the spring of the year 1300, to which the 

 historical allusions decidedly point. But in 1897 

 Sig. Angelitti, of the Naples Observator}-, pub- 

 lished a paper in which he showed that the posi- 

 tions of the moon, \'enus, and Saturn given in 

 the poem agree wonderfully well with their actual 

 places at the end of March, 1301. The greatest 

 difficulty to those who accept the year 1300 has 



