A little further on in the same treatise, Dante tells us the periods in which all these nine spheres revolve. It is an interesting passage, and illustrates the poet’s vivid way of bringing home to his own mind and those of his readers the effects of the different celestial motions.[594] The ninth sphere, which is the Crystalline or Primum Mobile, causes by its daily revolution the daily revolution of all the other spheres, which are within it; it is, as Beatrice says, “costui, che tutto quanto rape l’altro universo seco.”[595] Therefore if we imagine it possible for the motion of this Crystalline Heaven to cease, what will be the effect on the motions of the other heavens? Obviously their diurnal movement from east to west will cease, and we shall only see them revolving, each in its own period, from west to east. There will be no alternation of day and night, or rather, there will be one long day of six months, and one long night of the same length, in any given place all over the earth. In the same way the moon will be invisible for half the month; and all the planets will be invisible to us for half their periods; and as Dante believed only the known hemisphere of Earth to be inhabited, he could say that during one half of each period no one would see the planet at all. He enumerates the half period in each case, as follows:— ------------------------------------------------------------------------ [594] _Conv._ II. xv. 132-155. [595] “That one which sweeps the whole of the rest of the universe along with it.” _Par._ xxviii. 70, 71. ------------------------------------------------------------------------ _Half Period Whole Period given by Alfraganus_ “_Planets._” (_approximate_) (_El. Ast. ch. xvii._) _given by Dante. Years Months Days_ Saturn 14½ years 29 5 8 Jupiter 6 ” 11 10 14½ about. Mars about a year 1 10 22 nearly. Sun 182 days, 14 hrs. ... 365¼ nearly. {about the same Venus { as the sun. ... ” Mercury ” ” ... ... ” Moon 14½ days ... (nearly) 27, 7¾ hrs. (Paris edition, 29 days, 12¾ hrs.)