Reflecting Instrument - The Sextant

The Sextant

The main article, Sextant, covers the use of the instrument in navigation. This article concentrates on the history and the development of the instrument

The origin of the sextant is straightforward and not in dispute. Admiral John Campbell, having used Hadley's octant in sea trials of the method of lunar distances, found that it was wanting. The 90° angle subtended by the arc of the instrument was insufficient to measure some of the angular distances required for the method. He suggested that the angle be increased to 120°, yielding the sextant. John Bird made the first such sextant in 1757.

With the development of the sextant, the octant became something of a second class instrument. The octant, while occasionally constructed entirely of brass, remained primarily a wooden-framed instrument. Most of the developments in advanced materials and construction techniques were reserved for the sextant.

There are examples of sextants made with wood, however most are made from brass. In order to ensure the frame was stiff, instrument makers used thicker frames. This had a drawback in making the instrument heavier, which could influence the accuracy due to hand-shaking as the navigator worked against its weight. In order to avoid this problem, the frames were modified. Edward Troughton patented the double-framed sextant in 1788. This used two frames held in parallel with spacers. The two frames were about a centimetre apart. This significantly increased the stiffness of the frame. An earlier version had a second frame that only covered the upper part of the instrument, securing the mirrors and telescope. Later versions used two full frames. Since the spacers looked like little pillars, these were also called pillar sextants.

Troughton also experimented with alternative materials. The scales were plated with silver, gold or platinum. Gold and platinum both minimized corrosion problems. The platinum-plated instruments were expensive, due to the scarcity of the metal, though less expensive than gold. Troughton knew William Hyde Wollaston through the Royal Society and this gave him access to the precious metal. Instruments from Troughton's company that used platinum can be easily identified by the word Platina engraved on the frame. These instruments remain highly valued as collector's items and are as accurate today as when they were constructed.

As the developments in dividing engines progressed, the sextant was more accurate and could be made smaller. In order to permit easy reading of the vernier, a small magnifying lens was added. In addition, to reduce glare on the frame, some had a diffuser surrounding the magnifier to soften the light. As accuracy increased, the circular arc vernier was replaced with a drum vernier.

Frame designs were modified over time to create a frame that would not be adversely affected by temperature changes. These frame patterns became standardized and one can see the same general shape in many instruments from many different manufacturers.

In order to control costs, modern sextants are now available in precision-made plastic. These are light, affordable and of high quality.