Just as the time of day may be told, if one is accustomed to watch the sun, by noting how much of the daily course has been run, so the time of day or night (according to her phase) may be told by the moon; only with her we must take into consideration the rapid and variable motion eastwards in the zodiac. Alfraganus says that her _mean_ daily motion in longitude is 13° and nearly 11 minutes,[441] but that a small amount must be added or subtracted every day in order to find her true motion. As a clock, therefore, for use in daily life, she leaves much to be desired. This daily motion makes her fall constantly behind the sun, so that she loses time every day, and not even the same amount of time; for though she crosses the meridian on an average 50½ minutes later each day, the interval is sometimes only 38 minutes and sometimes as much as 66 minutes. And her times of setting and rising are even more variable. ------------------------------------------------------------------------ [441] It is easily seen that this is correct, if we divide the 360° of the moon’s path through the zodiac by the 27 days and eight hours in which she traverses it, and returns to the same star. Brunetto Latini says “La lune s’esloigne _dou Soleil_ chascun jor xiii degrez po s’en faut,” but he is wrong, having apparently forgotten that the sun is also moving in the same direction, at the average rate of nearly 1° daily. ------------------------------------------------------------------------ This is easily understood if we remember how much the sun’s time of setting varies throughout the year, according to the part of the zodiac in which he is travelling, and consider that what the sun does in a year the moon does in a month. And in her case the effect is sometimes exaggerated, sometimes diminished, for her path in the zodiac is inclined to the sun’s path; moreover, her greatest departure from it to north and south takes place in different parts of the zodiac at different times. These facts were well known to Ptolemy and to mediæval astronomers, and everyone who watches the moon must have noticed how variable are the intervals between one moonrise or one moonset and the next. The extent of variability also depends upon our latitude (just as with sunrises and sunsets); and in Florence the retardation in one day may sometimes be only twenty minutes, sometimes an hour and twenty minutes. The difference would be less in Dante’s Purgatory, since this was in latitude 32° south, and the intervals between moonset and moonset become less variable in length as we approach the equator, just as the days become less unequal all the year round. Still, they would vary a good deal, so we must conclude that Dante only means to indicate the time quite roughly when he uses the moon as a clock. As a matter of fact, he seldom does so in the _Divine Comedy_ without giving us another clue to the time as well.