In 1731, John Hadley, an Englishman, and Thomas Godfrey, in Philadephia, independently developed instruments for measuring angles by means of double reflection. These instruments were the forerunners of octants and, later, sextants.

The sextant serves to determine the altitude of a celestial body on the horizon or the angular distance between two celestial bodies. It was widely used on ships to calculate positions by astronomical methods. Some types of sextant were also used on land for surveying work.

The instrument is composed of a frame in the shape of a circle sector of approximately 60 degrees fitted with a fixed mirror consisting of a glass plate silvered on one half of its surface. On the vertex is hinged an alidade fitted with a second mirror, which ends in a nonius scale facing the instrument's graduated arc.

The first step in using the sextant is to aim at the horizon through a sight or small telescope. The observer slides the alidade by hand along the graduated scale until the image of a chosen celestial body—reflected in the moving mirror and then in the fixed mirror—lines up with the horizon viewed through the unsilvered half of the fixed mirror. With the nonius and graduated scale, the observer can then directly read the angle measuring the altitude of the body above the horizon.

The sextant was often fitted with moving color filters to be placed on the path of the light rays. This attenuated the brightness of the Sun and thus protected the observer from dazzle.