As formidable a piece of ironmongery as one would wish to encounter. In actual fact it is merely an instrument that measures the angle a heavenly body (star, planet, sun, moon) makes with the visible horizon.

It derives it's name from the arc at the bottom which is one sixth of a circle. The principles of a sextant are easy to master but its use requires some skill and practice. Small errors make for large discrepancies in one's position. <hrdata-mce-alt="Parts of the sextant" class="system-pagebreak" title="Parts of the sextant" />

Parts of the sextant

The sextant basically consists of a telescope, a half silvered horizontal mirror which the telescope "looks" through and a moving arm on which the index mirror is fixed. By manipulating this arm a star or other celestial body can be made to appear on the horizon. Accurate adjustments are made by means of a micrometer knob. The angle can then be read off the arc and micrometer. The shades are to use when the object being looked at is bright - such as the sun.

The trick is to make the celestial body just brush the horizon - and herein lies somewhat of a knack. <hrdata-mce-alt="Principle of the sextant" class="system-pagebreak" title="Principle of the sextant" />

Principle of the sextant

The sextant relies on the optical principle that if a ray of light is reflected from two mirrors in succession then the angle between the first and last direction of the ray is twice the angle between the mirrors. And this angle can then be read off the arc.

To use the sextant the telescope must be focused on the horizon. The celestial body to be shot, found and the sextant aimed at it. Bring the body down to the horizon by moving the arm along the arc and then clamp the arm. Using the micrometer knob make small adjustments while gently swaying the instrument slightly from side to side until the heavenly body just brushes the horizon.

When this is achieved instantly make a note of the time, seconds first, then minutes and hours, then the name of the body and its observed altitude. Every second of time counts - an error of 4 seconds equates to an error of a nautical mile in the position.

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