Navigation and Seamanship

2.2.3. Important parts of a sextant

1) Main frame on which the other parts are mounted.

2) Limb is the lower part of the frame and carries the arc graduated in degrees.

3) Index arm : Used to indicate the readings in degrees on the arc.

4) Micrometer drum is used to make find adjustment of the index arm.

5) Index mirror : mounted at the upper end of the index arm. It is perpendicular to the plane of the instrument.

6) Horizon glass : Also perpendicular to the plane, when the index arm is set to exactly zero degree, the horizon glass is parallel to the index mirror. The horizon glass is parallel to the index mirror. The horizon glass is divided into two halves vertically. The part nearest to the frame is silvered and the other half is clear optical glass.

7) Telescope : Mounted parallel to the plane.

8) Index shade glass : These glasses are used to dim the bright celestial bodies.