Sextant
source: britannica.com
“sextant, instrument for determining the angle between the horizon and a celestial body such as the Sun, the Moon, or a star, used in celestial navigation to determine latitude and longitude. The device consists of an arc of a circle, marked off in degrees, and a movable radial arm pivoted at the centre of the circle.”
Octant
source: amhistory.si.edu
“An octant is a portable instrument that uses a small mirror to bring two images together–those of the sun and the horizon, for instance–to determine latitude at sea by observing the altitude of celestial bodies. It has an arc of 45o or more that measures angles of 90o or more.”
Slawik, E., & Reichert, U. (1998). Atlas der Sternbilder: Ein astronomischer Wegweiser in Photographien [224 S]. Spektrum Akad. Verl. Heidelberg, S.68, 100.