The sextant constitutes the evolution and improvement of the octant, an invention of the English naval officer, John Campbell, in 1757. The improvement that the sextant brought about is that the octant has a scale of one eighth of the cycle, meaning 45 degrees and measures angles up to 90 degrees, whilst the sextant has a scale of one sixth of the cycle, meaning 60 degrees and measures angles up to 120 degrees.

The sextant consists of a fixed 45 degree arc, meaning one-sixth of the circumference of the circle, which is graduated per one-tenth of a degree, by a rotating arm and by a system of mirrors. The movable arm carries one of the mirrors and can rotate in relation to the fixed graduated arc, giving an image of the celestial body, the arc of which is measured when this image coincides with the zone of the horizon. When measuring the angle of the sun with the horizon, meaning its height, during its «meridian passage», that is when the sun is on the highest point of its orbit, that angle corrected by simple tables according to the time of the year, gives the latitude of the observer, which is observed directly in the determination of latitude. This angle is used in determining latitude.

In order to find longitude, precise time and complex calculations are needed using tables.

In order to protect the observer’s eyes from intense light of the sun, octants and sextants also have removable filters to reduce the brightness of light.