Mode of Calculating Time
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.
The divisions of the day; |
Again, he must necessarily understand the subdivisions both of the day and
the night, in order to know at what hour to
order the reveille, 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 are subtended by the diameter of
his are 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.