Sextant

Sextant, optical instrument used for the measurement of angular distance between any two objects. The sextant was invented independently, in about 1730, by both the English mathematician John Hadley and the American inventor Thomas Godfrey, its main use being in navigation. It enables a navigator to measure the angular elevation of the Sun and other celestial bodies, and from this information the navigator's latitude and longitude—that is, the navigator's location—can be determined.

The sextant's operation depends upon superimposition of the images of the two objects whose distance is being measured. This is achieved by means of an optical system consisting of a telescope and two mirrors, one fixed and one movable. In the accompanying diagram, the telescope T is mounted in a fixed position on the body of the instrument, pointing towards the mirror A. The top half of this mirror is transparent, the bottom half silvered. A second mirror, mirror B, is angled above mirror A. An observer looking through the telescope towards the horizon H sees the horizon through the unsilvered portion of mirror A and at the same time sees the image of the star or the Sun S on the silvered portion of mirror A, as rereflected from mirror B above. By moving B by manipulating the lever L, the image of the star is brought into coincidence with the image of the horizon. The angular distance between the star and the horizon can then be read on a scale engraved on the body of the sextant. This scale is an arc of one-sixth of a circle, or 60 degrees. Each degree on the scale of the sextant is equivalent to two degrees of angular distance between the objects actually observed because the light from S reflects off two mirrors.

When observations with sextants are taken on board ships at sea, the actual observed horizon can be used for measuring altitudes. On land this method of observation is seldom possible because of the irregularity of terrain. In this case a so-called artificial horizon is employed, consisting of a pool of mercury or some other horizontal reflecting surface. By observing both the star itself and the image of the star in the mercury, a sextant reading can be obtained that is equal to double the actual altitude of the star.

In aircraft and on ships in a rough sea, when the visible horizon is not a clearly defined line, bubble sextants or bubble octants are used. The octant resembles a sextant but has an arc of only one-eighth of a circle, or 45 degrees. In these bubble instruments, the bubble of a built-in spirit level is observed in place of the horizon.