Nocturnal with perpetual calendar (Nocturnal [Perpetual Calendar], c. 1745)

The Nocturnal (also: Star Clock) of the German Museum was made of ivory and wood. With it, time could be measured at night by determining the angle between a star and the polar star. In principle, the nocturnal is a representation of the apparent movement of the rear wheels of the constellation Kleiner or Großer Wagen around the Polarstern in 24 hours (actually: celestial pole) in the center. The instrument has three discs that can be adjusted. On the lower disk, the months with days (1-28/31) and the zodiac signs are indicated, on the upper rotatable can be set, among other things, the moon age (1-30). Above it is a small disc to which the long index arm is attached. For use, the instrument is held like a hand mirror on the handle made of ivory. An ivory plate embedded in the hand bears the engraving "BOTH-BEARS", which means that either the constellation Big Bear or the constellation Small Bear could be used for the correct angle adjustment on the firmament. Through the hole in the middle of the disk was targeted after the presetting of the disks of the Polarstern. If the finger of the index hands with the inscription "LB" (for Little Bear) or "GB" (for Great Bear) pointed at certain stars in the respective constellation, the instrument was correctly aligned. On the hour scale on its back, the time could then be read. (Author: Mareike Wöhler 2018)