A sextant is a doubly reflecting navigation instrument that measures the angular distance between two visible objects, most often a celestial body and the horizon, by bringing the reflected image of one into alignment with the direct view of the other in a pair of mirrors. The instrument's principle was first implemented around 1731, independently, by the English mathematician John Hadley and the American glazier Thomas Godfrey; both built working reflecting octants using the same double reflection principle Isaac Newton had described but never published decades earlier. Enlarged to a sixty degree arc reading up to one hundred and twenty degrees, the design became the standard tool of celestial navigation at sea for over two centuries.
Facts
Invented YearSourced to the subject's own account MeasuresSourced to the subject's own accountThe angular distance between two visible objects, most commonly a celestial body and the horizon, used to fix a position by celestial navigation. 1 Operating PrincipleSourced to the subject's own accountAn index mirror rotated on a graduated arc brings the reflected image of one object into alignment, in a half silvered horizon glass, with the direct view of a second object; the angle read off the arc at that alignment is the angular distance between them. 1 Operating RangeSourced to the subject's own accountGraduated to read angles up to about 120 degrees, twice the sixty degree arc that gives the instrument its name. 1 Open Questions
ResolutionNo single figure; typically on the order of a tenth of a minute of arc on a marine sextant, depending on the vernier or micrometer drum.
Not stated as a single fixed figure in the cited source; practical reading precision depends on the instrument's vernier or micrometer drum and the observer's skill, commonly on the order of a tenth of a minute of arc on a marine sextant, rather than one specified value. Classification
Instrument ClassSourced to the subject's own accountMeasurement or Metrology Instrument 1 Connections
Anticipated By
Invented By
Hadley's 1730 reflecting octant, developed independently of Thomas Godfrey's parallel invention, is the direct ancestor of the sextant.
Source John Hadley (Wikipedia)
Used In
Astronomy, Disciplines Celestial navigation fixes a position from the measured angle between a celestial body and the horizon.
Source Sextant (Wikipedia)
Sources
1. Sextant (Wikipedia)
WikipediaLead section
A sextant is a doubly reflecting navigation instrument that measures the angular distance between two visible objects.
History section
The principle of the instrument was first implemented around 1731 by John Hadley and Thomas Godfrey.
Lede, instrument class
A sextant is a doubly reflecting navigation instrument that measures the angular distance between two visible objects, most often a celestial body and the horizon, by bringing the reflected image of one into alignment with the direct view of the other in a pair of mirrors.
- Used In: Astronomy
View the Source John Hadley (Wikipedia)
WikipediaInvented By: John Hadley, Work sectionQuote, Invented By: John Hadley, Work section
In 1730 Hadley invented the reflecting octant. An American, Thomas Godfrey, independently invented the octant at approximately the same time.
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