The sextant is an instrument that measures the position of celestial bodies above the horizon and uses calculations to determine a position on Earth. The first hand-held sextant, based on an arc of 1/6 of a circle or 60 degrees, was developed in 1759. It was more accurate than the positioning instruments that came before, such as the astrolabe, because mirrors within the sextant allowed angles to be measured from the Earth’s horizon (or an artificial horizon used in conditions such as fog when the horizon was not visible) rather than the false “horizon” created by the position of the instrument itself.
Merriam-Webster Online Dictionary
sextant (noun) an instrument for measuring angular distances used especially in navigation to observe altitudes of celestial bodies (as in ascertaining latitude and longitude)