All the seven planets (that is, including sun and moon), are occasionally mentioned together, “tutti e sette,”[356] and their movement in the ecliptic is referred to in _Par._ x, 14: “L’obbliquo cerchio che i pianeti porta.”[357] In the 14th and 15th chapters of the second treatise of the _Convivio_, already so often quoted, Dante draws an elaborate comparison between the seven planets and the seven sciences of the Trivium and Quadrivium. The moon is Grammar, Mercury Dialectic, Venus Rhetoric, the Sun Arithmetic, Mars Music, Jupiter Geometry, and Saturn Astrology. The reasons given for the latter are the slow movement and the height (_i.e._ distance from Earth) of Saturn, which Dante compares with the length of the time taken in learning astronomy, and the loftiness of its subject. ------------------------------------------------------------------------ [356] “All the seven.” _Son._ xxviii. 14, and _Par._ xxii. 148. [357] “The oblique circle which carries the planets.” ------------------------------------------------------------------------ In the same way the star sphere is said to resemble Physics and Metaphysics, the Primum Mobile Ethics, and the Empyrean Theology. The argument is very fanciful, but just what would appeal to readers of Dante’s day, who loved to find allegories everywhere; and it gives him an opportunity of instructing them very simply in his beloved science. He sometimes uses the planetary periods as divisions of time (as Plato said they should be used), for computing earthly events, and in him this does not seem affectation, as it would with almost anyone else. It seems quite natural that Cacciaguida, when speaking in the heaven of Mars, and answering a question regarding the date of his birth, should count the time not by solar but by Martian years, saying that from the beginning of the Christian Era to the day of his birth, Mars had returned to his Lion (the constellation of Leo) five hundred and eighty times.[358] The length of the Martian year according to Alfraganus is 1 Persian year, 10 months, and 22 days nearly. The Persian year (as he tells us in the first chapter of the _Elementa_) consists of 12 months of 30 days each plus 5 extra days, making 365 days exactly (not 365¼ days like the Roman), so the Martian year is 687 days (the modern estimate is 686·9). 687 days × 580 gives us 1091 a.d. as the date of Cacciaguida’s birth. This is consistent with the date of his death, for he had just told Dante that he fell in the Crusade to which he followed the Emperor Conrad, and we know that this was fought in the year 1147.