Instruments for the Determination of Longitude and Latitude

Celestial Starglobes

The Octant

The octant was developed simultaneously by John Hadley (1682-1744), who demonstrated it to the Royal Society in 1731, and by Thomas Godfrey (1704-49), a glazier and self-taught astronomer from Philadelphia. It was the first instrument to measure the height of a star or the sun by using mirrors to bring a reflected image of the object alongside the horizon when viewed through the sight. This development greatly improved the accuracy of observations. As its name implies, an octant forms an eighth of a circle, but the use of reflection doubles the angle measured to a maximum of about ninety degrees. The octant was also known as Hadley's quadrant, after John Hadley. Octants were made and used until the 19th century.

The Sextant

The sextant also takes its name from its shape – a sixth of a circle. Astronomical sextants were in use by the 16th century, but the marine version was developed in about 1757 by Captain John Campbell with the help of the London instrument maker, John Bird (1709-76). The sextant worked on the same principle as the octant. Having a longer scale than the octant, it could be used to measure angles up to 120 degrees. Following its introduction, the sextant became one of the principal navigational instruments on ships throughout the world and is still used today. A quintant performs the same functions, but has a slightly longer scale.