we must turn to thirteenth-century Ristoro of Arezzo, who has set forth in detail the results which he believed would follow from a change in the ecliptic. If all the seven planets moved in the same narrow path, there would be much less variety in their “aspects,” and therefore in their influences upon the earth; and in the frequent eclipses one would prevent the other from looking at the earth (“impedimenterebbe l’uno l’altro a guardare la terra”), and so hinder its action. And since it is the northward movement of the sun which causes our plants to blossom, and later to bear fruit, if there were no such motion we should see no renewal of life. He argues further that if the obliquity of the ecliptic were greater, countries in high latitudes would have too severe a winter to be inhabited; while if it were less, the summer there would not be warm enough for the ripening of harvests; therefore in whichever way it differed from the actual value, a smaller part of the earth would be habitable. “Or ti riman, lettor, sopra il tuo banco, Dietro pensando a ciò!”[396] ------------------------------------------------------------------------ [396] “And now remain, reader, on your bench, thinking over this!” _Par._ x. 22, 23. ------------------------------------------------------------------------ How much of this was in Dante’s mind it is hardly possible to say, but it is evident that the “via tortuosa” of the seven planets, and its effects on the earth, was a favourite subject for thought in his time.[397] ------------------------------------------------------------------------ [397] Thus Petrarch: “Le stelle vaghe e lor viaggio torto.” (“The wandering stars and their winding way.”) _Sonetto de Morte di Madonna Laura._ ------------------------------------------------------------------------ But the most famous passage relating to the sun’s path and its aspect in strange latitudes is _Purg._ iv. 52-84. Here Dante is able at once to understand the startling appearance of the sun travelling to his left instead of his right, as seen on the first morning in Purgatory, for he grasps the full meaning of Virgil’s explanation, and caps it, in a way which seems astonishingly quick-witted to unastronomical readers! The two poets had begun to climb the mountain just at sunrise, and sat down to rest on a ledge at about half-past nine, looking east. Dante at first looked down at the shore, whence he had so painfully climbed, then raised his eyes to the sun, and was amazed to find that its rays struck him on the left shoulder.