Nocturnal, or Nocturlabe as they were sometimes called, was first described in 1272 as a means of calculating the time at night.
How it works
The Nocturnal works on the principle that stars close to the Celestial Poles are circumpolar.
Nocturnal consists of several pieces of metal or wood, which are attached at the
center so they can rotate relative to one another. At the axis of rotation is a hole.
in use, the Nocturnal is held upright by the handle until the Polaris
be sighted through the hole. The long arm of the device is then turned until it
lies along the line made by the two brightest stars in the constellation known
as the Ursa Major. These two stars are often used as "pointers"
because they are easily seen and they lie along a line which passes close to the
Polaris. The bright star in the Ursa Minor can be used in the same
Ursa Minor is used, the inner dial would be turned so that the pointer
marked "LB" would lie against the date on which the observation is
being made. By doing this, the correction from sidereal time to solar
time is automatically corrected.
setting the inner dial to the correct date, the Polaris is sighted
through the hole and the long arm is turned until the bright star in Ursa
Minor lies on it. The time is then read off from the scale on the central
dial -- just as if the long arm were acting as the hand of a clock.
the Ursa Major is chosen as a reference, the procedure is the same,
except that the small pointer marked "GB" is set against the date.