The sextant is a scientific instrument used to measure and construct angles. Precise, light, and easy to transport, it is mainly used at sea to measure the angular distances and altitudes of stars. This allows the observer to determine their location by making a calculation using an astronomical "ephemeris" (a table published in nautical almanacs), at the times given by chronometers. These measurements were required to create nautical charts.
It has a metal frame in a circular sector shape, starting at a 60° amplitude. On the front is an embedded silver limb on which the alidade slides to rotate around the center of the limb's circumference. There are mirrors perpendicular to the limb: one is part of the alidade and the other is part of one of the frame's arms. The other arm holds an astronomical eyepiece. The movement of the eyepiece in parallel with the instrument was devised in 1790 by Spanish Navy General José de Mazarredo. He suggested it to British manufacturer John Bird (1709–76), who then became the first to introduce this improvement to the sextant.
This one was manufactured by the London firm Garth.