NOCTURNAL, Nocturalibus, an instrument chiefly used at sea, to take the altitude or depression of some stars about the pole, in order to find the latitude and hour of the night.
Some nocturnals are hemispheres, or planispheres, on the plane of the equinoctial. Those commonly in use among seamen are two; the one adapted to the polar star, and the first of the guards of the little bear; the other to the pole star, and the pointers of the great bear.
This instrument consists of two circular plates, applied to each other. The greater, which has a handle to hold the instrument, is about 2½ inches diameter, and is divided into twelve parts, agreeing to the twelve months; and each month subdivided into every fifth,
Nocturnal fifth day; and so as that the middle of the handle
Nodes. corresponds to that day of the year wherein the star
here regarded has the same right ascension with the
sun. If the instrument be fitted for two stars, the
handle is made moveable. The upper left circle is
divided into twenty-four equal parts for the twenty-
four hours of the day, and each hour subdivided into
quarters. These twenty-four hours are noted by
twenty-four teeth to be told in the night. Those at
the hours 12, are distinguished by their length.
Plate CCI. In the centre of the two circular plates is adjusted a
fig. 2. long index, moveable upon the upper plate. And
the three pieces, viz. the two circles and index, are
joined by a rivet which is pierced through the centre
with a hole, through which the star is to be observed.
To use the nocturnal, turn the upper plate till the
long tooth, marked 12, be against the day of the
month on the under plate: then, bringing the in-
strument near the eye, suspend it by the handle with
the plane nearly parallel to the equinoctial; and
viewing the pole star through the hole of the centre,
turn the index about, till, by the edge coming from
the centre, you see the bright star or guard of the
little bear, (if the instrument be fitted to that star):
then that tooth of the upper circle, under the edge of
the index, is at the hour of the night on the edge of
the hour-circle: which may be known without a light,
by counting the teeth from the longest, which is for
the hour 12.