nocturnal

nocturnal, an early instrument designed essentially for measuring the time of night by means of Polaris and either the ‘Pointers’ of the Plough in the constellation Ursa Major, or the Guards of the constellation Ursa Minor. It was first described in print by Michel Coignet in 1581. The earliest nocturnals consisted of two concentric circular plates of brass or wood. The circumference of the larger was divided into twelve equal parts corresponding to the months of the year and that of the smaller into 24 equal parts corresponding to the hours of the day. The larger plate carried a handle and the inner plate was fitted with a long index, one end of which was pivoted to the centre of the plate. To ascertain the time by the nocturnal a projecting tooth at the position corresponding to 12 o'clock on the smaller plate was turned to coincide with the date on the larger plate. The instrument was then held at arm's length and Polaris viewed through a small hole at the centre of the instrument. The index arm was then turned until the bevelled edge coincided with the line joining the ‘Pointers’ of Ursa Major, or the Guards of Ursa Minor as the case might be, and the time of night was then read off the scale of hours on the smaller plate.

Nocturnals were also used for working out the time of high water for which the establishment of the port was known. It was not too difficult a matter to relate the time of the moon's southing and the age of the moon by means of circular scales attached to the nocturnal. By applying the establishment of the port to the time of the moon's southing, the approximate time of high water, or ‘full sea’ as it was called, could readily be found.