sextant

sextant
Public domain · source
Name	Sextant
Caption	Marine sextant
Classification	Navigational instrument
Invented	18th century
Inventor	John Hadley; Thomas Godfrey
Related	Octant; Astrolabe; Chronometer

sextant A sextant is a precision navigational instrument used to measure the angle between two visible objects, typically a celestial body and the horizon, for the purpose of determining position at sea. It is historically central to maritime navigation alongside instruments such as the chronometer, compass, and log and has been used in voyages by figures like James Cook and institutions such as the Royal Navy. The device influenced exploration during periods including the Age of Discovery and innovations in astronomy by observers from institutions like the Royal Observatory, Greenwich and the United States Naval Observatory.

History

The development of the sextant emerged from earlier instruments including the astrolabe, the quadrant, and the octant. Independent inventors such as John Hadley in England and Thomas Godfrey in the American colonies contributed to its 18th‑century refinement. The sextant was rapidly adopted by navies and merchant services including the Royal Navy, the United States Navy, and the East India Company for long‑distance voyages to regions like Cape Horn, Cape of Good Hope, and the South Pacific Ocean. Its role became pivotal during expeditions by explorers such as James Cook, George Vancouver, and scientific voyages sponsored by institutions like the Royal Geographical Society and the British Admiralty.

Design and components

A typical instrument comprises a graduated arc, an index arm, a frame (often brass), mirrors (index and horizon), a telescope, a vernier or micrometer, and a handle. The graduated arc is commonly a 60° sector derived from designs used in the octant, while optical components trace lineage to devices studied at the Royal Observatory, Greenwich and the workshops of instrument makers like John Bird. Instruments often bear makers’ marks from firms such as R. M. Young Company and historical firms like Frodsham and Troughton & Simms. Accessories include sighting scopes, sun filters (sunglass shades used during observations linked to practices by James Cook), spirit levels, and protective cases used by organizations like the National Maritime Museum.

Principles of operation

The sextant measures the apparent angular separation between two objects by using plane mirrors to bring the images into coincidence. A user aligns a celestial body such as the Sun, Moon, Polaris, or a planet observed by institutions like Lick Observatory with the visible horizon as adopted in practices from the Royal Observatory, Greenwich. Combined with time obtained from a chronometer or time services such as United States Naval Observatory time signals, the observed altitude is reduced via sight reduction tables developed by institutions including the U.S. Naval Observatory and the Her Majesty's Nautical Almanac Office to derive lines of position and fix latitude and longitude.

Types and variations

Variants include the marine sextant, the reflecting quadrant, the mirror sextant used in aviation, and large surveying sextants used for geodetic work by agencies like the Ordnance Survey. Specialized forms include the bubble sextant for use without a natural horizon in aircraft and spacecraft programs such as those of NASA and the Royal Air Force. Handheld versions were used by explorers from the Antarctic expeditions led by Ernest Shackleton and Robert Falcon Scott, while high‑precision laboratory sextants supported observatories including Greenwich and Paris Observatory.

Usage and navigation techniques

Navigators use the instrument to take sights of the Sun at local apparent noon for latitude, of Polaris for northern hemisphere fixes, and of planets or stars listed in the Nautical Almanac for celestial navigation. Techniques include the noon sight, running fix, intercept method developed by Marcq Saint‑Hilaire‑style reductions, and altazimuth reductions taught in institutions such as the United States Naval Academy and the Royal Navy training establishments. Data from a sextant is combined with chronometer time, dead reckoning maintained by logs and compasses like those from Danforth, and charting on charts produced by agencies such as NOAA and Admiralty charts to plot a vessel’s position.

Accuracy, errors, and calibration

Sources of error include instrument index error, collimation error, atmospheric refraction, dip of the horizon depending on eye height above sea level, and human error in timing and observation. Corrections are applied using tables from the Nautical Almanac and procedures defined by naval standards such as those of the Royal Navy and United States Navy. Calibration and adjustment require bench testing against fixed references, reference stars observed at observatories like Greenwich and verification against radio navigation systems such as LORAN and GPS where available. Skilled observers from maritime services and surveying agencies can achieve arcminute accuracy sufficient for safe navigation.

Modern adaptations and applications

Although electronic systems like Global Positioning System receivers have largely supplanted celestial navigation for routine use, the sextant remains a backup and training instrument used by the Royal Navy, United States Coast Guard, and private yachtsmen in races such as the America's Cup. Adaptations include optical coatings, micrometer drum verniers, and ergonomic materials developed by instrument makers collaborating with institutions such as NASA for bubble sextants on early space missions. Sextants are preserved and exhibited by museums including the National Maritime Museum, the Smithsonian Institution, and the Science Museum, London and continue to be taught at maritime academies and by organizations like the Royal Yachting Association.

Category:Navigational instruments