Marine sextant parts mariner Sextant errors marine sextant


Marine Sextant is an instrument which is used to measure altitudes of celestial bodies, that is it is used to measure angles. Sextant has 1/6th arc of a circle, which is 60 degrees. But this instrument is able measure angles to about 120 degrees due to the principle of double reflection.

Principle of sextant:

There are two principles 

  • When a ray of light is reflected by a plane mirror, the angle of incidence is equal to the angle of reflection when they (Incident Ray, Reflected Ray & the Normal) are in the same plane
         that is , Angle of Incidence = Angle of Reflection
  • When a ray of light suffers two successive reflections in the same plane by two plane mirrors, the angle between the incident ray and the final reflected ray is twice the angle between the mirrors. This double reflection causes the sextant to measure 120 degree.

Measurements by a sextant:

  • Altitude of celestial bodies.
  • Vertical sextant angles(VSA) of terrestrial objects.
  • Horizontal sextant angles(HSA) of terrestrial objects.
Both VSA &HSA are used in position fixing using terrestrial objects.

Parts of sextant:




  • Index mirror with shades
  • Horizon glass with shades
  • Telescope
  • Frame
  • Micrometer
  • Graduated arc(60 degree) and Index bar.
Errors of sextant:

  • Adjustable errors
  • Non-Adjustable errors
  1. Adjustable Errors: These are errors which can be adjusted by the Officer using sextant.
  • Index Error: When index bar is at zero, the horizon glass not parallel to the index glass.
  • Side Error: Horizon glass is not perpendicular to the plane of the instrument.
  • Error of Perpendicularity: Index glass is not perpendicular to the plane of the instrument.
  • Error of Collimation: Telescope not being parallel to the plane of the instrument.(Not applicable to the latest Sextant)
    2. Non - Adjustable Errors: These are errors which can only be adjusted by shore assistance.

  • Graduation Error: It is due to inaccurate graduation in the micrometre of sextant.
  • Shade Error: It happens when the colour shades are not parellel to each other.
  • Centering Error: The arc of circle on which graduated arc is part of is not accurately centered.
  • Optical Error: Errors due to problems with the glasses.
  • Back-lash leading to inconsistent errors:It occurs due to micrometre movement due to wear on rack and worm.