Sextant

Original content by: Dayne Rugh

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About This Artifact

The Sextant, its name derived from the Latin meaning one sixth of an arc of a circle, is one of the most important nautical instruments that sailors and mariners relied upon for years, well before the creation of the first GPS systems.  Nowadays, getting directions, finding locations, and knowing where you are geographically is a fairly simple task when aided by the internet, online mapping systems (such as Google Maps), and GPS systems.  For sailors of the past, however, determining your location was a much more difficult task.  It required quite a bit of mathematical know-how, a steady hand, and if you were sailing, a steady ship (which was often quite difficult to come by when rocking back and forth at sea).  People have been sailing the world's oceans for thousands of years, and the invention of instruments such as the sextant made it easier for explorers to reach places across the globe.  Instead of pulling out a smartphone or a computer to determine location, sailors relied on the ancient art of celestial navigation, with the sextant eventually becoming one of their main navigational tools.

The first of these instruments to be invented can be credited to Englishman John Bird, a mathematical instrument maker, in 1757.  Although Bird invented the sextant, it was based off models of the octant by John Hadley and Thomas Godfrey, who in turn actually based their work off of Isaac Newton, who came up with the theory behind the instrument in 1699.  In order for a sailor to determine how far north or south he was when out at sea, he needed to know his latitude.  In order to determine latitude, a sailor was required to measure an angle between an object in the sky, usually the sun or the North Star, and the horizon.  Before the sextant existed, there were many other less advanced instruments that could measure this angle, but they often would lead to less accurate results.  Therefore, Isaac Newton came up with what is known as "double reflection".  Simply put, Newton's theory, when combined with Hadley and Godfrey's invention lead to Bird's creation of the sextant.  This instrument, through the use of a series of mirrors and lenses, allows the user to almost have the magical ability of seeing two different things (in this case, the sun and the horizon), while using just one eye.

By looking through the main eyepiece of the sextant, you'll see a window with two sides - one of glass, and one mirrored.  After positioning the sun on the mirror side, you turn the knob that moves the arm of the sextant, which will make the sun descend in the window.  Keeping your eye on the sun while continuing to turn the knob, you'll find that the sun will actually hit the horizon in the window.  Once this occurs, take a look at where the arm has stopped on the curved arc, and read the angle on which it has landed.  This does not tell you the latitude, but is part one of a larger equation.  It may sound complicated at first, but it's actually fairly simple.  Sextants are still in regular use today (as of 2014, they can still be found on United States Naval Warships), and while we have become so heavily dependent on electronics, knowing how to operate one of these instruments is incredibly helpful to navigators for when power is lost or batteries die!

Questions for Further Thought

  1. Do you think that the invention of electronic GPS has made the use of sextants and other tools such as written maps obsolete? Why or why not?
  2. Today,what professions might still use sextants regularly? Can you think of any professions that would benefit from their use?
  3. Back in the 18th century, the British government offered huge sums of money to people who could invent instruments that aided in navigation. Why do you think the government was motivated to offer such large monetary prizes for these inventions?