Navigation and Seamanship

2.2.5. Sextant altitude

2.2.5.Uses of Sextant

  

The sextant is used for measuring the angle at the observer’s eye between.

(i) The top of some object such as lighthouse, tower etc. and the waterline (horizon) when the sextant is held vertically. This is called Vertical Sextant Angle (VSA).

(ii) Two objects on different bearings on shore with the sextant held horizontally is called Horizontal Sextant Angle (HAS).

(iii) A celestial object and the horizon.

  Sextant altitude

     It is the angular height of an object, above the visible horizon or as read from the arc of a sextant, before correction for index error if any.  

Observed altitude:

            This is the angular height of an object above the visible horizon as read from the arch of the sextant and correction for index error. When measuring Vertical Sextant Angle (VSA) of an object set the index at 0 and look at the top of the image down to coincide with the water line directly below it.

The following formula are used to calculate the distance between the vessel, from which the sextant angle is taken and the structure (light house/church/chimney/ any structure on shore with known height),   

Height of the object  in meters

Distance in nautical miles = ----------------------------------------------------------

                                                          1852 x tan VSA

Height of the object in feet

Distance in nautical miles = ----------------------------------------------------------

                                                        6080 x tan VSA