An instrument chiefly used at sea, to take the altitude or depression of some of the stars about the pole, in order to find the latitude and the hour of the night.
·noun An instrument formerly used for taking the altitude of the stars, ·etc., at sea. II. Nocturna...
Webster's Dictionary of the English Language
See nocturnal arc. ...
The Sailor's Word-Book
That part of a circle, parallel to the equator, which is described by a celestial object, between it...