The Marine Sextant
John Bird's Sextant
A sextant is an instrument for measuring the altitude of the sun or another celestial body and in some cases the term is generally used to include related devices (Columbia Electronic Encyclopedia, 2015). Furthermore, throughout history, the terms astrolabe, quadrant, backstaff, octant, and sextant are at times used interchangeably (Blewitt, 1995; Ifland, 2003; Janus, 2010) and can result in some confusion. For the purpose of this discussion, the term sextant will refer to the modern sextants first designed in 1759, by John Bird (Ifland, 2003). Just before that, the first octants appeared simultaneously in 1731, John Hadley in England, and Thomas Godfrey in Philadelphia, both were doubly reflecting instruments (Columbia, 2015; Ifland, 2003). The octants were based on 1/8th of a circle but moving the arm one degree moved the altitude measure two degrees. John Bird’s sextant and the modern sextants as we know them today are based on 1⁄6 circle and thus measure 120 degrees, giving a greater measure than the octant. (Blewitt, 1995; Bowditch, 1975; Ifland, 2003); moving the index arm one degree results in a two degree difference in the angle of the sun (Blewitt, 1995). The octant and sextant allowed users to shoot stars and moon using the reflection of light in mirror (Ifland, 2003), and to measure precise latitude. (Blewitt, 1995; Bowditch, 1975). The precision with which the sextant could measure distances and altitudes, would help to revolutionize navigation, although at that time of its introduction, there were still many different older tools still being used.
How it Works
The sextant works based on the principle that a reflected ray of light leaves a plane surface at the same angle at which it strikes the surface, or “the angle of reflection is equal to the angle of incidence” (Bowditch, 1975) and the principle that two reflective planes or
mirrors create an angle of inclination which, through logic and transposition, when
doubled, equals the altitude of the celestial body:
2BGC = BDC with line DC the observer’s line of sight
(Bowditch, 1975).
2BGC = BDC with line DC the observer’s line of sight
(Bowditch, 1975).
Rather than attempting to look into the sun the user places a sunshade on
the sextant and the line of sight is pointed at the horizon directly below the sun, then the index arm is pulled down until the sun is reflected right at the horizon line in the center (Bowditch, 1975). This simultaneously solved two problems: it eliminated the risk of blindness caused by shooting the sun and it solved the difficulty of sighting stars at night by optimizing the light through reflections.
While using the sextant is relatively simple, the calculations necessary to determine latitude, at times other than high noon or by Polaris, become more complex and still rely on dead reckoning, or deduced reckoning, and involve an incredibly large base of knowledge, which now includes coordinates of longitude and latitude, steering the true course by a compass, and use of estimated speed, distance and time to plot a course with reasonable accuracy (Chapman, 1970 pp337(j)337(p)).
While using the sextant is relatively simple, the calculations necessary to determine latitude, at times other than high noon or by Polaris, become more complex and still rely on dead reckoning, or deduced reckoning, and involve an incredibly large base of knowledge, which now includes coordinates of longitude and latitude, steering the true course by a compass, and use of estimated speed, distance and time to plot a course with reasonable accuracy (Chapman, 1970 pp337(j)337(p)).