Item History: The sextant, an instrument for measuring angles, was developed from a suggestion by Captain John Campbell of the Royal Navy in 1757. Those promoting the use of lunar distances, or "lunars," for finding longitude at the end of the 18th century stimulated the invention of the sextant. The sextant was designed to find longitude by measuring the angular distance between the moon and a nearby star. The octant, already invented in 1731, could only measure up to 90 degrees of angle. The new "lunar method" for finding longitude, however, required the ability to measure angles up to 120 degrees. It is called a sextant because its arc spans 60 degrees, or one sixth of a circle, but the double reflection of the image of the celestial body enables the sextant to measure angles up to 120 degrees. Like the octant, the sextant was used to measure the altitude of a body (sun, moon, stars, planets) above the horizon, as well as the angular distance between two bodies. Because of its ability to measure larger angles the sextant gradually replaced the octant. It has found widespread use for measurement of latitude by measuring the “altitude” (i.e., height of a star) above the horizon.
To use a sextant it is held vertically. The index arm is set to zero and the reflection of the celestial body being measured is viewed in the horizon mirror. The index arm is rotated until the reflected image of the celestial body is aligned with the horizon and the angle of the celestial body above the horizon is read from the scale.
A sextant has two mirrors, a horizon mirror and an index mirror. The horizon mirror is half silvered so that the horizon can be viewed through the clear section and the reflection of the celestial body can be viewed reflected in the silvered portion. The index mirror is attached to the index arm so that it can be rotated to reflect the image of the celestial body into the horizon mirror and then into the eye. The lower end of the index arm rides over the arc of the sextant that contains the scale marked off in degrees to indicate the angle of the celestial body above the horizon.
Modern sextants can read the angle to a 0.1 minute level of accuracy, i.e. one-600th of a degree or one-tenth of a mile. In practice, actual accuracy to one-half mile is acceptable and quite good. The usual standard is accuracy to within five miles. The sextant (or octant) is meant to get the ship across the ocean. Once near the coast (20-100 miles) the more accurate techniques of piloting are relied upon for a safe landfall. The sextant is still the standard instrument for taking the observations required for celestial navigation.