The motion of those stars which neither rise nor set, but are always seen circling round the pole, is clearly described. In the heaven of the sun, spirits gather round Dante and Beatrice, and circle round them, “Come stelle vicine ai fermi poli;”[262] and the motion of the stars near the pole is said to be slow, like the part of a wheel which is near the axle.[263] We may compare these descriptions with the statement of Alfraganus:—“Eæ stellæ (i.e. the northern circumpolar) vertuntur omnes circa idem punctum. Et quei ex iis puncto huic est vicinior, minorem conficit circulum: motusque ejus appâret lentior.”[264] Dante assumes that his readers understand this motion when he describes himself as seeing unknown stars near the South Pole. The Mount of Purgatory is supposed to be situated in the southern hemisphere, and at his first arrival, before dawn, he turns to the right, after looking at Venus in the east, therefore to the south, and sees near the pole four stars so brilliant that the whole sky seems to rejoice in their radiance, and he pities the “widowed” northern hemisphere because they are invisible there.[265] In the evening, as soon as it begins to grow dark, although he is engaged in an interesting discourse with a friend with whom he had exchanged warm greetings, he looks eagerly once more towards the pole (note the intensity expressed by the repeated “pure”)—
“Gli occhi miei ghiotti andavan pure al cielo, Pur là dove le stelle son più tarde, Sì come rota più presso allo stelo.”[266]
He now sees three bright stars which make this pole glow with light, and Virgil says that the stars he saw in the morning are low, while those now visible have risen higher to take their place.
“Ed egli a me: Le quattro chiare stelle Che vedevi staman son di là basse, E queste son salite ov’ eran quelle.”[267]
The diurnal movement of the starry heaven is also alluded to in the Convivio, and contrasted with that immensely slow movement which was discovered by Hipparchus, and is called by us Precession. The starry heaven, says Dante, displays one of its poles to us, and keeps the other hidden; and in like manner it displays only one movement to us, and keeps the other almost hidden. By the first, it revolves once in every day from east to west; the other is nearly insensible, being only one degree in a hundred years, and it is from west to east.[268] This is the value of precession as given by Alfraganus, following Ptolemy.
It was this stupendous and mysterious cycle which was used by Dante to measure the age of Beatrice, though for his own he used (as we saw) the ordinary measure of the sun’s period. When he first saw “la gloriosa donna”[269] she had only been in this life so long that the starry heaven had moved towards the east one-twelfth part of a degree. Therefore, she was one-twelfth of a hundred years old, or 8 years 4 months, “so that it was near the beginning of her ninth year that she appeared to me, and I saw her nearly at the end of my ninth.”[270]
Dante tells us how many stars had been counted by “the sages of Egypt,” by whom he means Ptolemy and the other Alexandrians: he did not know that the star catalogue of the Almagest was originally made by Hipparchus of Rhodes.
“Dico ch’ il cielo stellato ci mostra molto stelle; chè, secondochè li savi d’Egitto hanno veduto, infino all’ultima stella che appare loro in meridie, mille ventidue corpora di stelle pongono, di cui io parlo.”[271]
It will be noted that he carefully avoids saying that this is the number of all the stars visible from the whole earth. Had he, like Ristoro, run away with the idea that a blank space on the globe meant a blank in the sky, his night sky seen from Purgatory would have been strangely bare and dull! Yet both writers used the same text-book. Dante is here following Alfraganus closely, for he had written in his nineteenth chapter:—
“Sciendum itaque sapientes inivisse mensuram stellarum fixarum omnium, quoad instrumentis observari eæ potuerunt, extremam usque meridiei partem, in tertio climate ipsis conspicuam.... Stellæ universae quarum agi mensura potuit sunt mille viginti duæ.”[272]