Nocturnal (instrument)

A nocturnal is an instrument for telling the time at night from the position of a star relative to the pole star, working because the fixed stars appear to circle Polaris once every twenty-three hours and fifty-six minutes. To use it, the observer sighted Polaris through a hole at the instruments center and rotated a pointer arm to line up with a chosen reference star, usually one from the Big Dipper or Little Dipper, and read the time off an inner hour dial, correcting for the season with an outer dial marked by month. The instrument, known formally as the horologium nocturnum, is credited to the ninth-century figure Pacificus of Verona, though the earliest surviving picture of one in use dates only to the twelfth century, and it reached a wider audience after Martin Cortes de Albacar described it in his 1551 navigation manual Arte de Navegar; it only works in the northern hemisphere, where the stars wheel predictably around the pole star.