The nocturnal or nocturlabium is used to determine the time at night with the help of the pole star (alpha UMi) and the star Kochab (beta UMi). Both stars belong to the constellation of the Little Bear. Some instruments also use the two stars Dubhe (alpha UMa) and Merak (beta UMa) from the constellation of the Great Bear, which point to Polaris, to determine the time.
The present instrument is set up for Kochab (beta UMi) but also makes it possible to aim for the two stars of the Great Bear.
The use of the nocturnal is shown on the back, based on a woodcut from the work Cosmographicus liber Petri Apiani Mathematici studiose collectus, Landshut 1524. The date is set with the long spike of the serrated disc. Then lift the instrument so that the polar star can be seen through the sighting hole in the centre. The long pointer is now pointed at the star Kochab and the time (local time) is read within the circle of prongs or felt in the darkness: this is the purpose of the prongs.
The complete description of the instrument as PDF for download
