AB and CD are two mirrors fixed, as in the diagram, parallel to each other and facing each other, i.e., the reflection on CD is seen on AB. A small telescope and a vernier travelling over a scale, graduated in degrees, constitute the sextant.
The sextant is fixed firmly to view the horizon, when the reading on the scale is zero, i.e., the reading when the horizon is seen through both the mirrors.
The arm of the sextant is a rotated until the reflected image of the sun is seen while the horizon is still seen directly, as in figure.
Then, the angle between the horizontal and CE is twice the angle through which the arm is rotated. Since the graduations on the scale is double the angle through which the arm is rotated, the direct setting gives the reading straightaway.
