Nocturnal (instrument)

[1] Sometimes called a horologium nocturnum (time instrument for night) or nocturlabe (in French and occasionally used by English writers), it is related to the astrolabe and sundial.

The actual horologium nocturnum, a precursor for the later nocturnal instruments, was invented in the 9th century by Pacificus of Verona.

[2] Even if the nightly course of the stars has been known since antiquity, mentions of a dedicated instrument for its measurement are not found before the Middle Ages.

[5] It was described also c. 1530 by Petrus Apianus in his Cosmographicus Liber, republished later by Gemma Frisius with a widely circulated illustration of the instrument while being used by an observer.

Similarly, it is not possible to determine longitude unless the observer also knows the standard time from a chronometer.

A nocturnal as costume jewellery. This is a functioning nocturnal, though only about 5 cm tall. It shows the month ring on the outside in brass. The silver coloured inner disk shows the time and has an indicator on one edge. By setting the indicator to the month and day (in this case, a few days into October), centring Polaris in the hole in the middle and rotating the pointer attached to the centre to a specified circumpolar star, the arm indicates the time (in this case, 8 pm).
A nocturnal made in Vienna.
Operating scheme of a nocturnal.