An early instrument designed for measuring time of night by means of Polaris (the Pole Star) and other points in constellations. It was first described in 1581. Nocturnals could also be used to work out the time of high water at a port for which the "establishment" was known. Establishment refers to the interval between the time of meridian passage of the new or full moon and the time of the following high tide. A constant for a given port.