The art and science of determining a vessel's position by measuring the altitudes of celestial bodies with a sextant.
Quick facts
Regulation
IMO MSC.232(82)
In practice
The practical celestial observation sequence begins with measuring the sextant altitude of a body, noting the precise chronometer time, correcting the raw altitude for index error, dip, semi-diameter, and atmospheric refraction, and then entering the Nautical Almanac and sight reduction tables to compute the intercept and azimuth. Two or more simultaneous position lines from different bodies give a celestial fix. A noon sight of the sun provides latitude directly and, combined with longitude from a forenoon and afternoon sight, yields the ship's noon position. Celestial navigation remains the essential backup when electronic systems fail, and regular practice during sea passages maintains officer proficiency.
Regulatory detail & full definition
Celestial navigation is the art and science of determining a vessel's position at sea by observing the altitudes of celestial bodies—the sun, moon, planets, and stars—with a sextant and reducing those observations to position lines using sight reduction tables or a calculator. Bowditch (American Practical Navigator) is the standard reference work for the technique, covering the theory of the celestial sphere, the use of the Nautical Almanac for extracting body coordinates, and the intercept method for plotting position lines. IMO MSC.232(82) on ECDIS performance standards and STCW Table A-II/1 both retain celestial navigation as a required competency for the officer in charge of a navigational watch.
The practical celestial observation sequence begins with measuring the sextant altitude of a body, noting the precise chronometer time, correcting the raw altitude for index error, dip, semi-diameter, and atmospheric refraction, and then entering the Nautical Almanac and sight reduction tables to compute the intercept and azimuth. Two or more simultaneous position lines from different bodies give a celestial fix. A noon sight of the sun provides latitude directly and, combined with longitude from a forenoon and afternoon sight, yields the ship's noon position. Celestial navigation remains the essential backup when electronic systems fail, and regular practice during sea passages maintains officer proficiency.