nocturnal

A nocturnal is an instrument used to determine the local time based on the relative positions of two or more stars in the night sky. 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. Knowing the time is important in piloting for calculating tides and some nocturnals incorporate tide charts for important ports. 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. The earliest image presenting the use of a nocturnal is in a manuscript dated from the 12th century. Raymond Lull repeatedly described the use of a sphaera horarum noctis or astrolabium nocturnum. With Martín Cortés de Albacar's book Arte de Navegar, published in 1551 the name and the instrument gained a larger popularity. 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.