Sextant is called so, its graduated arc is about one sixth of a circle.
Principle – If a ray of light reflected twice of in the same
plane by 2 plane mirrors; The angle between the first and last direction of ray
is twice the angle between mirrors.
Parts of Sextant
Arc – Metal frame on edge.
Index bar – Arm which can rotate about the center of the arc.
Index Glass – Mirror on a mount of index bar over the center of arc.
Horizon Glass – Mirror mounted on a small frame which perpendicular to main frame (left half is unslivered).
Arc is graduated in degrees, and so arranged that when the index glass is parallel to the horizon glass, an index on the index bar should point to zero on the scale.
Telescope carried on a collar fitted on the index bar.
There is a Vernier scale for finer readings, it also consist
of a scale which slide along the arc.
Notes on Sextant
- Test for Perpendicularity, Side Errors and Index ERROR before taking sight.
- Ensure non of the screws are LOOSE.
- The glasses get coated with moisture ESPECIALLY after sunset.