Sextant Standard Pattern

The device consists of an arc of a circle, marked  off in degrees, and a movable radial arm pivoted at the centre of the circle. A telescope, mounted rigidly to the framework, is lined up  with the horizon. The radial arm, on which a  mirror is mounted, is moved until the star is  reflected into a half-silvered mirror in line with  the telescope and appears, through the  telescope, to coincide with the horizon. The angular distance of the star above the horizon is  then read from the graduated arc of the  sextant. Form this angle and the exact 160mm  radius, designed to give maximum accuracy.