Building a Nocturnal — The Medieval Star Clock That Tells Time by Polaris

A nocturnal (also called a horologium nocturnum) is a handheld instrument that tells the time at night by measuring the position of the pointer stars of Ursa Major (the Big Dipper) as they rotate around Polaris, the North Star. The stars appear to revolve around the celestial pole once every 23 hours and 56 minutes (a sidereal day), and their angular position at any moment — combined with the date — reveals the local time. The nocturnal consists of two concentric discs: an outer disc graduated with the months and days of the year, and an inner disc graduated with the hours. A long pointer arm pivots at the centre. The observer holds the instrument at arm's length, sights Polaris through the centre hole, and swings the pointer arm to align with the guard stars of Ursa Major. The hour is then read where the pointer crosses the hour scale. Nocturnals were standard navigation instruments from the 13th through 17th centuries, carried by every ship's pilot alongside the cross-staff and compass. This blueprint builds a functional nocturnal from brass or stiff card.