The sextant was designed to find longitude by measuring the angular distance
between the moon and a nearby star. It was also used on land and to find
latitude at sea. A sextant is similar to an octant, but more powerful and
precise. The arc extends 60o or more and measures angles of 120o
or more. Sextants were traditionally made of brass, with scales graduated on
silver. Modern sextants are often made of plastic. Most have telescopic sights.
In 1675 King Charles founded the Royal Observatory so astronomers could take precise measurements of the orbit of the moon in relation to fixed stars. The hope was that once the movements of the celestial bodies were properly catalogued, longitude could be found by taking three simultaneous observations of the distance between the moon and the sun or a fixed star, the altitude of the moon, and the altitude of the sun or star. The lunar distance method was extremely difficult to develop, however, because the moon’s orbit is irregular, the stars’ positions, which would be used to mark time, were difficult to precisely identify, and an instrument that could accurately take the necessary measurements was yet to be made. The first suitable star catalogue was published in 1725, but it wasn’t until the mid-eighteenth century that the lunar tables were sufficiently tested and put to use with octants and sextants. Early examples of sextants from the 1750s were large, heavy, and costly, but the invention of the dividing engine in 1777 led to the production of smaller and less expensive instruments.
Sextants designed for aircraft navigation are equipped with a pendulum or a
gyroscope that serves as an artificial horizon, as well as a mechanism that
allows the navigator to average several observations taken in rapid succession.
Ref: "Sextant" and "Aircraft Sextant" in Robert Bud and
Deborah Warner, eds., Instruments of Science. An Historical Encyclopedia
(New York & London, 1998), pp. 530-534.
C. H. Cotter, A History of the Navigator’s Sextant (Glasgow, 1983).
Howes, Derek, "The Lunar Distance Method of Measuring Longitude" in
William J.H. Andrewes, ed., The Quest for Longitude, (Cambridge: 1996),