Navigation and Seamanship

2.2.1. Marine sextant

 

Sextant is an optical instrument used for measuring angles between any two points horizontally and vertically. The first instrument developed for measuring the altitude of celestial body while at sea was “CROSS-STAFF”. The disadvantage of this was that the user to look at the horizon and at the body at the same time. In 1590, the ‘BACKSTAFF’ or ‘Davis’s quadrant’ was invented by John Davis , which was the advanced stage of cross – staff. Today’s sextant is an instrument is an instrument designed to permit measurement of the angle between the lines of sight to two objects with great precision. It derives its name form the fact that its arc is approximately one-sixth of a circle, he cause of its optical principle it can measure angles upto to about 120^o or twice the value of the arc itself. In 1730, optical principle was realized and sextant was developed by Sir Hadley in England. Sextant can be used to measure the attitudes of celestial bodies or vertical angle of any terrestrial body. In spite of the modern electronic navigational equipments, sextant is still retained in the vessels due to its simplicity and reliability. It is extensively used on all medium and large vessels doing long coastal navigation and astronomical navigation.