Display 6: Octant

In the 18th century, the octant marked a significant advance in maritime navigation. This instrument allowed for measuring the altitude of celestial bodies much more accurately than earlier devices such as the Jacob's staff or the Davis quadrant. The octant owes its name to its shape: an arc of one-eighth of a circle (45°), which, thanks to mirror reflection, allowed for measuring angles of up to 90°.
The octant was developed almost simultaneously around 1730 by the Englishman John Hadley and the American Thomas Godfrey. The instrument used two mirrors: a fixed horizon mirror and a movable index mirror. By aligning the celestial body with the horizon through reflection, the observer could determine the angle between the two very accurately.

As with the Davis Quadrant, the user did not look directly at the sun. Combined with tinted lenses, this made the octant safer for the eyes and suitable for regular measurements. Measurements were taken around local noon for the sun, or at night for stars and planets.
The octant's greater accuracy was due to its mirror technology and finer scales. However, the instrument remained susceptible to errors, such as mirror deviation and index errors, which required correction by experienced mariners.
The octant was the direct precursor to the sextant, which was introduced later in the 18th century and would eventually become the standard instrument for celestial navigation at sea.