Expeditions needed to know where they were located and where they were going. Navigators would take their position on a regular basis, especially when ships headed into open seas or entered parts of the world that were far from familiar landmarks. The 18th century saw the introduction of advanced equipment that assisted navigators. These instruments were often made from solid materials like brass but were based on ancient practices of celestial navigation using the Sun, Moon, and stars.
One of the earliest navigational instruments was known as the quadrant. It was designed as a wedge-shaped scale that formed 1/4 of a full circle or 90 degrees. The angle of the North Star Polaris over the horizon was taken by lining the star up with sights on the edge of the quadrant. A plum bob suspended from the quadrant would then hang down over a marked point to give a vertical measurement of degrees latitude.
Although Sir Isaac Newton had written about angle-measuring technology, it was during the 18th century that practical, working versions of navigational instruments were produced. In the 1730s, two independent inventors, one an American and one an English mathematician, developed the octant, which used 1/8 of a full circle or 45 degrees as its arc. The octant worked with the doubly reflecting mirrors that would later be used in the sextant.
The sextant is an instrument that measures the position of celestial bodies above the horizon and uses calculations to determine a position on Earth. The first hand-held sextant, based on an arc of 1/6 of a circle or 60 degrees, was developed in 1759. It was more accurate than the positioning instruments that came before, such as the astrolabe, because mirrors within the sextant allowed angles to be measured from the Earth’s horizon (or an artificial horizon used in conditions such as fog when the horizon was not visible) rather than the false “horizon” created by the position of the instrument itself.
Navigators looked through an eyepiece to “take a sighting” with the sextant. Some sextants used a mirror to reflect the horizon as one half of the view for the sighting and the celestial object as the other. Another sextant model used a mirror to reflect the horizon alone. A movable arm, also fitted with a light-reflecting mirror, could then be adjusted along the instrument’s arc to superimpose the view of the celestial body over the view of the horizon. With this “sighting,” a position could be calculated.