Nautical Sextants

Sextants have been the primary device for offshore navigation for centuries, initially developed to guide mariners on trade route passages. While they are used to measure angles between a celestial body and the horizon, they can also be used to measure the apparent height of a known landmark for determining range, or to measure the angle between two landmarks to establish a line of position.

How does it works?
A sextant is a measuring instrument used to measure the angle of elevation of a celestial object, traditionally the Sun above the horizon. Knowing the angle and time of day, traditionally mid-day for the sun, one can calculate the degree of latitude. A sextant's view merges two views. One view is of the sky, through the mirrors. The other view is of the horizon. One uses a sextant by adjusting the arm and a worm adjustment until the lower edge of an image of a celestial body touches the horizon

Accuracy of a sextant
A sextant's accuracy is expressed in "seconds of arc". Each minute of angular measurement represents a distance of one nautical mile, so sextants can generally read out to one-fifth or one-tenth of a minute. From a small boat platform, most navigators can expect an accuracy of within a few miles.

Types of sextants