Origins

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.

A 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.

When in use, the Nocturnal is held upright by the handle until the Polaris can 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 way.

If 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.

After 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.

If 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.

Remarks

The Nocturnal could only be used in the northern hemisphere because it requires the user to be able to see Ursa Major or Ursa Minor, which lie near the North Celestial Pole.