It is time, indeed, which rules all human action and especially the affairs of war.
So that a general must be familiar with the dates of the summer and winter solstices, and the equinoxes, and with the rate of increase and decrease of days and nights between these; for by no other means can he compute correctly the distances he will be able to traverse either by sea or land.
He must also be acquainted with the subdivisions of day and night so as to know when to sound the revally and to be on the march;
for it is impossible to obtain a happy end unless the beginning is happily timed.
Now for the time of day there is nothing to hinder our observing it either by the shadow or by the sun's course or by his position and height in the heavens, but it is difficult to tell the hour of the night, unless one is familiar with the system and order of the twelve signs of the Zodiac in the starry sky, knowledge of which it is quite easy to gain by studying the constellations.
For since, though nights are of unequal length, yet during the course of every night six out of the twelve signs of the Zodiac must appear above the horizon, it follows of necessity that equal parts of the twelve signs must appear at the same times of the night.
As the position each day of the sun in the Zodiac is known, it is evident that at his setting the part diametrically opposite must rise.
So that the portion of the night which is past is to be judged by the portion of the Zodiac which has risen after this;
and the number and size of the signs of the Zodiac being known, the subsequent subdivisions of the night correspond to them.
On cloudy nights, however, we must observe the moon, because as a rule, owing to her size, her light is visible in whatever part of the heaven she may be situated.
We can guess the hour at times from the time and place of her rising and at times again from those of her setting,
if here too we have sufficient previous knowledge to be familiar with the daily difference in the hour of her rising.
Here also there is an easy method of reckoning, for the period of her revolution is generally speaking one month, and all the months are similar as far as we can perceive.
Walbank Commentary