This instrument was made especially for René Descartes (1596-1650). The Frenchman resided in the Dutch Republic from 1629 to 1650, during which time he conducted experiments and wrote his most important philosophical works. He could use this convenient pocket instrument on his travels. To determine the time at night, the nocturnal (or nocturlabe) had to be aligned with the Pole Star. It also houses a sundial and a compass.