5.4 KiB
| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Reflecting instrument | 3/4 | https://en.wikipedia.org/wiki/Reflecting_instrument | reference | science, encyclopedia | 2026-05-05T09:41:54.244254+00:00 | kb-cron |
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.
== Types of sextants == While most people think of navigation when they hear the term sextant, the instrument has been used in other professions.
Navigator's sextant The common type of instrument most people think of when they hear the term sextant. Sounding sextants These are sextants that were constructed for use horizontally rather than vertically and were developed for use in hydrographic surveys. Surveyor's sextants These were constructed for use exclusively on land for horizontal angular measurements. Instead of a handle on the frame, they had a socket to allow the attachment of a surveyor's Jacob's staff. Box or pocket sextants These are small sextants entirely contained within a metal case. First developed by Edward Troughton, they are usually all brass with most of the mechanical components inside the case. The telescope extends from an opening in the side. The index and other parts are completely covered when the case cover is slipped on. Popular with surveyors for their small size (typically only 6.5–8 cm [2+1⁄2–3+1⁄4 in] in diameter and 5 cm [2 in] deep), their accuracy was enabled by improvements in the dividing engines used to graduate the arcs. The arcs are so small that magnifiers are attached to allow them to be read. In addition to these types, there are terms used for various sextants. A pillar sextant can be either:
A double-frame sextant as patented by Edward Troughton in 1788. A surveyor's sextant with a socket for a surveyor's staff (the pillar). The former is the most common use of the term.
== Beyond the sextant ==
=== Quintant and others === Several makers offered instruments with sizes other than one-eighth or one-sixth of a circle. One of the most common was the quintant or fifth of a circle (72° arc reading to 144°). Other sizes were also available, but the odd sizes never became common. Many instruments are found with scales reading to, for example, 135°, but they are simply referred to as sextants. Similarly, there are 100° octants, but these are not separated as unique types of instruments. There was interest in much larger instruments for special purposes. In particular a number of full circle instruments were made, categorized as reflecting circles and repeating circles.
=== Reflecting circles ===