A nocturnal is a device used to determine local time based on the relative positions of two or more stars in the night sky. Sometimes called a horologium nocturnum (nighttime timepiece) or nocturlabe (in French and occasionally used by English writers), this tool is related to the astrolabe and sundial.

Typically constructed of materials like wood or brass, a nocturnal consists of two discs—one outer disc marked with the months of the year and an inner disc marked with hours (and possibly half-hours or quarter-hours on larger models) as well as locations for one or more reference stars. It also features a rotating pointer on the same axis as the discs.

The fundamental concept behind the nocturnal is to provide local time based on the time of year and the observation of specific stars, such as the North Star. In the northern hemisphere, all stars appear to revolve around the North Star during the night, and their positions can be used to determine the time.

The inner disc is adjusted so that the mark for the chosen reference star aligns with the current date on the outer disc. The North Star is sighted through the center of the device, and the pointer is rotated to point at the chosen reference star. The intersection of the pointer with the hour markings on the inner disc indicates the local time.

This instrument has practical applications, especially in navigation, where knowing the time is crucial for calculating tides.

It’s a fascinating example of how people in the past used celestial observation and engineering to measure time during the night.

The term gained popularity through Martin Cortés de Albacar’s book “Arte de Navegar,” published in 1551, and the nocturnal has served as a kind of analog nighttime clock.


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