Navigation and Seamanship

2.2.2. Working principle of sextant

When a ray of light suffers reflection by two mirrors in a same plane, the angle between the original incident ray and the final emergent ray is twice the angle between the mirrors. The optics of sextant are based on the system of double reflection, in that the image of the body is reflected from the upper or index mirror to the lower or horizon mirror and then into the field of view of sextant telescope, where it is brought into co-incidence with the sea horizon which is seen through the clear portion of the horizon mirror.