In all human undertakings opportuneness is the most important thing, but especially in operations of war. Therefore a general must have at his fingers’ ends the season of the summer and winter solstice, the equinoxes, and the periods between them in which the days and nights increase and diminish. For it is by this knowledge alone that he can compute the distance that can be done whether by sea or land. Again, he must necessarily understand the subdivisions both of the day and the night, in order to know at what hour to order the reveillé, or the march out; for the end cannot be attained unless the beginning be rightly taken. As for the periods of the day, they may be observed by the shadows or by the sun’s course, and the quarter of the heaven in which it has arrived, but it is difficult to do the same for the night, unless a man is familiar with the phenomenon of the twelve signs of the Zodiac, and their law and order: and this is easy to those who have studied astronomy. For since, though the nights are unequal in length, at least six of the signs of the Zodiac are nevertheless above the horizon every night, it is plain that in the same portions of every night equal portions of the twelve signs of the Zodiac rise. Now as it is known what portion of the sphere is occupied by the sun during the day, it is evident that when he has set the arc subtended by the diameter of his arc must rise. Therefore the length of the night is exactly commensurate with the portion of the Zodiac which appears above the horizon after sunset. And, given that we know the number and size of the signs of the Zodiac, the corresponding divisions of the night are also known. If however the nights be cloudy, the moon must be watched, since owing to its size its light as a general rule is always visible, at whatsoever point in the heaven it may be. The hour may be guessed sometimes by observing the time and place of its rising, or again of its setting, if you only have sufficient acquaintance with this phenomenon to be familiar with the daily variation of its rising. And the law which it too follows admits of being easily observed; for its revolution is limited by the period of one month, which serves as a model to which all subsequent revolutions conform.