This instrument is really the reproduction of a 17th century Octant. Despite its poor precision in measuring the height of the sun and stars, it allowed navigators to determine geographic latitude.
In 1731 J. Hadley (England) and T. Godfrey (USA) independently proved the Octant. The arch of the Octant is 45º or 1/8 of the circumference (hence its name). The Sextant (1757) replaced the wooden frame with a metal one, progressively replaced the Octant and continues to be used.