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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Reflecting instrument | 4/4 | https://en.wikipedia.org/wiki/Reflecting_instrument | reference | science, encyclopedia | 2026-05-05T09:41:54.244254+00:00 | kb-cron |
The reflecting circle was invented by the German geometer and astronomer Tobias Mayer in 1752, with details published in 1767. His development preceded the sextant and was motivated by the need to create a superior surveying instrument. The reflecting circle is a complete circular instrument graduated to 720° (to measure distances between heavenly bodies, there is no need to read an angle greater than 180°, since the minimum distance will always be less than 180°). Mayer presented a detailed description of this instrument to the Board of Longitude and John Bird used the information to construct one sixteen inches in diameter for evaluation by the Royal Navy. This instrument was one of those used by Admiral John Campbell during his evaluation of the lunar distance method. It differed in that it was graduated to 360° and was so heavy that it was fitted with a support that attached to a belt. It was not considered better than the Hadley octant and was less convenient to use. As a result, Campbell recommended the construction of the sextant. Jean-Charles de Borda further developed the reflecting circle. He modified the position of the telescopic sight in such a way that the mirror could be used to receive an image from either side relative to the telescope. This eliminated the need to ascertain that the mirrors were precisely parallel when reading zero. This simplified the use of the instrument. Further refinements were performed with the help of Etienne Lenoir. The two of them refined the instrument to its definitive form in 1777. This instrument was so distinctive it was given the name Borda circle or repeating circle. Borda and Lenoir developed the instrument for geodetic surveying. Since it was not used for the celestial measures, it did not use double reflection and substituted two telescope sights. As such, it was not a reflecting instrument. It was notable as being the equal of the great theodolite created by the renowned instrument maker, Jesse Ramsden. Josef de Mendoza y Ríos redesigned Borda's reflecting circle (London, 1801). The goal was to use it together with his Lunar Tables published by the Royal Society (London, 1805). He made a design with two concentric circles and a vernier scale and recommended averaging three sequential readings to reduce the error. Borda's system was not based on a circle of 360° but 400 grads (Borda spent years calculating his tables with a circle divided in 400°). Mendoza's lunar tables have been used through almost the entire nineteenth century (see Lunar distance (navigation)). Edward Troughton also modified the reflecting circle. He created a design with three index arms and verniers. This permitted three simultaneous readings to average out the error. As a navigation instrument, the reflecting circle was more popular with the French navy than with the British.
=== Bris sextant ===
The Bris sextant is not a true sextant, but it is a true reflecting instrument based on the principle of double reflection and subject to the same rules and errors as common octants and sextants. Unlike common octants and sextants, the Bris sextant is a fixed angle instrument capable of accurately measuring a few specific angles unlike other reflecting instruments which can measure any angle within the range of the instrument. It is particularly suited to determining the altitude of the sun or moon.
=== Surveying sector === Francis Ronalds invented an instrument for recording angles in 1829 by modifying the octant. A disadvantage of reflecting instruments in surveying applications is that optics dictate that the mirror and index arm rotate through half the angular separation of the two objects. The angle thus needs to be read, noted and a protractor employed to draw the angle on a plan. Ronalds' idea was to configure the index arm to rotate through twice the angle of the mirror, so that the arm could then be used to draw a line at the correct angle directly onto the drawing. He used a sector as the basis of his instrument and placed the horizon glass at one tip and the index mirror near the hinge connecting the two rulers. The two revolving elements were linked mechanically and the barrel supporting the mirror was twice the diameter of the hinge to give the required angular ratio.
== References ==
== External links == National Maritime Museum Portrait of a merchant navy captain holding a Caleb Smith Octant.