Nocturnal (instrument)
instrument used to determine the local time based on the relative positions of two or more stars in the night sky
A nocturnal is an instrument used to determine the local time based on the position of a star in the night sky relative to the pole star. As a result of the Earth's rotation, any fixed star makes a full revolution around the pole star in 23 hours and 56 minutes and therefore can be used as an hour hand. The 4-minute difference between the solar day and sidereal day requires a correction of this giant clock based on the date of observation, and nocturnal helps to apply this correction.
Sometimes called a horologium nocturnum (time instrument for night) or nocturlabe (in French and occasionally used by English writers), it is related to the astrolabe and sundial. Knowing the time is important in piloting for calculating tides and some nocturnals incorporate tide charts for important ports.
History
The actual horologium nocturnum, a precursor for the later nocturnal instruments, was invented in the 9th century by Pacificus of Verona.
Even if the nightly course of the stars has been known since antiquity, mentions of a dedicated instrument for its measurement are not found before the Middle Ages. The earliest image presenting the use of a nocturnal is in a manuscript dated from the 12th century. Raymond Lull repeatedly described the use of a sphaera horarum noctis or astrolabium nocturnum.
With Martín Cortés de Albacar's book Arte de Navegar, published in 1551 the name and the instrument gained a larger popularity.
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The citation trail is more important than the brevity of the summary: it shows where individual claims can be examined in context. The source revision retrieved here is dated Sep 2, 2026. The linked authority identifier is Q1607611. None of the 0 selected statements returned an explicit reference. The first chronological checks are 1551.
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This entry incorporates text from “Nocturnal (instrument)” on English Wikipedia. Contributors are listed in the page history. Text is available under the Creative Commons Attribution-ShareAlike 4.0 License. Selected authority identifiers and statements are retrieved from Wikidata under CC0; their references and qualifiers remain part of the verification path.