This may be calculated by the shadow which the moon casts upon a
sun-dial, it being only necessary to know the moon’s age, which may be
found in an almanack. If the new moon happens in the morning, this day
is taken into the account; but if it happens after noon, the following
day is counted the first. The moon’s age is to be multiplied by four
and divided by five. The quotient must either be added to the hour,
which the shadow indicates on the sun-dial, and the sum will give the
time sought; or subtract from the quotient the hour shown by the moon
upon the dial, and the remainder will give the hour sought. The first
is to be done when the shadow falls on an hour of the afternoon, and
the latter when it falls upon an hour of the forenoon. The following
are examples:
1st. Suppose the moon to be ten days old, and the shade cast by the
moon upon the sun-dial to be at half-past two; or, that the shadow
cast by the moon falls on the place at which the shadow cast by the
sun stands at half-past two;—what o’clock was it then? The answer is
calculated as follows:—The moon’s age, 10 days × 4 = 40 40/5 = 8.
Eight, therefore, is the time when the moon was in the meridian, and 8
+ 2½ = 10½, or half-past ten, the hour sought.
2d. Suppose the moon to have been 18 days old, and the shadow cast by
it on the sun-dial to have marked eleven. This time is subtracted from
the hour when the moon was in the meridian on that day, and from which
the hour marked by the shadow must be deducted. The shadow shows here
11 o’clock in the forenoon, or one hour before noon, which, deducted
from 2h. 24m. gives 1h. 24m.; 2⅖ - 1 = 1⅖, or 24 minutes past one
o’clock.