Sextant

The marine sextant is the handheld instrument that measures the angle between a celestial body and the horizon, the sextant altitude Hs, to a precision of about 0.1 to 0.2 arcminutes. Its index mirror and fixed horizon glass bring the body’s reflected image down to the horizon; double reflection gives an arc graduated to 120 degrees over a frame of 60 degrees (one sixth of a circle, hence the name). A micrometer drum or older vernier reads the fine arc. The navigator removes index error first, then applies dip, refraction, and where needed semidiameter and parallax to reach Ho. The sextant also measures horizontal and vertical angles for terrestrial fixing.