The octant is a reflective instrument used to measure the height from the horizon of a reference star (the Sun or the Polar Star), in order to determine the latitude of a given point on the earth's surface. Generally it is formed of a frame shaped like the section of a circle  frame, with graduated measurements along the lower edge and an opening of about 45 ° (an eighth of an angle of turn).

Hadley's octant takes the form of the model exhibited after 1760. Octants were not only slowly adopted in Italy because they were very expensive.