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History of the sextant
Posted by      11/Feb/2022 14:55:40     Instruments    Comments 0
History of the sextant

The history of the sextant started in the 17th century and was developed from instruments such as the cross-staff and the Davis quadrant or backstaff. However, these instruments did not use reflection with the exception of the Spiegelboogh of Joost van Breen’s from around 1660 [1] and Hooke’s ‘reflecting instrument’ from 1666 [2]. Both instruments used a single reflection rather than a double reflection. The first double reflection instrument was invented by Sir Isaac Newton (1642-1727) who presented his design in 1699 to the Royal Society [3]. The documentation on this invention was, however, misplaced by its then secretary, Sir Edmond Halley, and only found again 1743 [4].

Godfrey and Hadley

Until 1730 there was hardly any development. In that year in the United States (then a British colony) an instrument based on double reflection and invented by Thomas Godfrey (1704-1749) was tested [5]. In 1731 the Royal Society gave a description of two instruments based on double reflection, developed by John Hadley (1682-1744) [6]. The first instruments by Hadley and Godfrey were similar in design and operation to that developed earlier by Newton. Both deviate from the ‘modern’ sextant in that they have the mirror and index arm parallel. The second instrument as invented by Hadley, is similar to the modern sextant with an index arm at right angles to the telescope. From that perspective it seems defendable that Hadley is the inventor of the modern sextant even though Godfrey and Newton invented an instrument based on double reflection before his invention.

Hadley’s second instrument became known as ‘Hadley’s quadrant’ and had a sector of 45°. Hadley himself referred to this instrument as an octant [7] which is correct as it can measure angles of up to 90° [8]. Hadley’s quadrant or octant was primarily used in astronomical positioning as a replacement to the Backstaff or Davis quadrant and thus for determining the latitude.

Reflection circle

In order to measure the longitude, two methods were under development at that time. One was the chronometer by John Harris, the other the lunar distance method. Both were based on the determination of the exact time of a location in relation to a standard time (at Greenwich for the British). Although there is some argumentation that both methods were competing this is not true. The chronometer is an excellent instrument to measure change in time. The lunar distance method was an excellent method to determine absolute time after a series of measurements and tedious computations. The two methods therefore support each other with the lunar distance method determining an absolute time difference and the chronometer keeping it (for some time). In the lunar distance method the angle between the moon and another heavenly body is determined requiring both the theoretical angular distance as well as an actual measurement of the same distance. When Tobias Mayer developed accurate tables for computing the angular distance and from those the longitude it was quickly found that for the actual measurement the octant with its 90° of arc was unsuitable. Tobias Mayer developed a repeating circle to overcome this problem.

The instrument works along the same principles as the sextant but uses a full circle rather than a 90° sector. Tobias Mayer’s design from 1754 [9] was constructed in 1757 by renowned instrument maker John Bird in 1757 as a reflecting circle with a radius of 16” (around 40 cm)5. The instrument was tested by (later admiral) John Campbell at sea and found to be unwieldy. At his request John Bird created a brass sextant of 120° in 1759 [10]. This instrument was found to be very suitable and the development of the reflecting circle was stopped in England as a result. In France the reflecting circle was further developed by Jean-Charles de Borda and Etienne Lenoir into a definite type in 1777. An instrument that was further developed was the repetition circle by Borda (and Lenoir) for land surveying. As that instrument does not use double reflection it is not discussed in this article.

From vernier to drum

Both the brass sextant and wooden octant remained largely unchanged until the early 20th century. The instruments had different types of frames, including the famous 'double frame' of Troughton and Simms, and the scales would be made of various materials. The least expensive instruments kept their bone / ivory scales whereas the more expensive instruments would go from brass through silver to platinum scales. But they invariably used vernier scales with a tangent screw. At the end of the 19th century the regular tangent screw with a limited movement would be replaced by the endless tangent screw allowing easier small adjustments using a toothed index scale. Fine reading was still done through a vernier set-up though. Around 1907 C. Plath [11] developed the first drum micrometer sextant. With this design the teeth of the index scale were cut to such an accuracy that the rotation of the worm indicated exactly 1 degree. The division of the degree would now be read directly on a drum fitted to the worm screw. In some sextants the drum would be augmented by a small vernier allowing even finer readings.

Towards the end

The drum was essentially the last invention of the sextant to reach production. Over the  years variations on the sextant were created but all used essentially the same set-up. The only real exception was the Freiberger Skalen sextant [12] which used a moving glass index rather than a brass one. One of the last developments was the Observator Mk IV [13] with the filters fitted inside the telescope allowing a gradual change of the filter. However, this was still evolution rather than revolution. The real revolution was to come from the digital sextant with a computer for performing the calculation from astronomical sight to position. A number of prototypes [14] were developed and one seems to have found its way into production [15] but in general the sextant has been all but replaced by GNSS positioning. The main use for the, usually still traditional drum, sextant is with yacht owners wishing a back-up in case of a power failure.


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