Introduction.
The marine sextant is a precision built optical instrument f which is used to measure altitudes and angles. It used to be an essential part of a ship’s navigation equipment. The sextant got its name from the fact that the length of its graduated arc is approximately one sixth of a circle. One of the optical laws upon which the operation of the sextant is based states that, when a ray of light is twice reflected by two mirrors in the same plane, the angle between the first and the last rays is twice the angle between the two mirrors (see drawing and explanation at the end of this section). As the sextant’s arc is one sixth of a circle or 60° of arc, it follows that the sextant can measure angles of twice this amount or 120°. In practice most manufacturers increase the length of arc to slightly more than one sixth of a circle and the modern sextant is usually graduated from 0° to 125° “on” the arc. The graduations are also continued over a 5° arc on the opposite side of the zero. This is called the “arc of Excess” and angles read on this part are said to be “off” the arc.

Micrometer sextant.