nocturnal

A simple instrument for telling the time at night by observation of the position of the two stars in the constellation Ursa Major known as the Pointers. The continuation of the line joining these stars passes very close to the north celestial pole, and can effectively be used as a giant clock-hand in the sky as the Earth's rotation causes them to sweep out a circle daily. The nocturnal consists of two concentric discs and a sighting arm, fixed at the centre by an eyelet through which the Pole Star can be sighted. The lower disc is graduated with the days of the year and the upper one with the 24 hours of the day. In use, the sighting arm is aligned with the Pointers. The dials are needed to convert the direct measurement, which is effectively the sidereal time, to mean solar time.