Inventor: John Campbell

The sextant is a device used for measuring angles between distant objects or heavenly bodies. The sextant was actually based on a long line of nautical instruments for angle measurement, but had some significant advantages. First, the sextant allowed measurements to be taken while only looking in one direction instead of lining up two separate sights. And second, the sextant could be used to determine longitude as well as latitude.

The devices preceding the sextant were designed for measuring latitude, which was usually determined by the altitude of the north star above the horizon. Navigation was based on this single measure: to sail to a specific port, navigators would sail north or south until they reached the latitude of that port. They would then sail east or west to reach it. Devices for measuring the north star’s altitude, such as the cross-staff or the Davis quadrant, often required the user to look in two directions: both at the star and the horizon. This was often difficult at sea.

To solve this problem a new device, the octant, was developed independently by John Hadley and by Thomas Godfrey. This device used two mirrors to allow the user to measure an angle while looking through a single eyepiece. The first mirror was stationary, and only half of it was silvered. The second mirror rotated and was attached to a scale with marked angles. The horizon was viewed through the non-silvered half of the first mirror, and the second mirror was rotated until the north star’s reflection in the other half was near the horizon. The north star’s altitude could then be read on the scale.

In the 1700’s, astronomers developed predictions of the angular distance between the moon and certain stars. This could be used to tell the time which was necessary to determine the longitude. However, the octant, like the earlier instruments, was designed for measuring latitude, which only required angles between 0 and 90 degrees. Angles greater than 90 degrees often had to be determined. Because of this, John Campbell created the sextant, an instrument that extended the range to 120 degrees, making it possible to determine both latitude and longitude. Most modern sextants would be more accurately called quintants or quadrants because they often measure even greater angles.

The sextant was and continues to be an important tool for both land and sea navigation. Although many improvements have been made to it, such as lighting systems for use at night or artificial horizons for use on land, the basic design has remained the same. Only recently has GPS begun to replace this device as a better navigation system.

Picture from Encarta Encyclopedia

Sources:

Ifland, Peter. "The History of the Sextant". 3 Oct. 2000. 2 Jan. 2004. <http://www.mat.uc.pt/~helios/Mestre/Novemb00/H61iflan.htm>

• This was a lecture given by Dr. Peter Ifland about the history of the sextant.

Cardoza, Rod. "Evolution of the Sextant". Nautical Brass Magazine Online. <http://home.earthlink.net/~nbrass1/cardart.htm>

• This website describes the history of the sextant.