Octant (Instrument)

The octant, also called the reflecting quadrant, is a navigation instrument that uses mirrors to measure the angle between a celestial body and the horizon, its name coming from the Latin for an eighth part of a circle because its arc spans one eighth of a circle while its mirrors double the measured angle to read a full quadrant. Two inventors developed it independently around 1730, the English mathematician John Hadley and the Philadelphia glazier Thomas Godfrey, though Hadley received greater credit because of Londons standing in scientific instrument making at the time. Because it let an observer see the horizon and a celestial object at once, reducing alignment error, and because it could be built about half the size of the earlier Davis quadrant with no loss of accuracy, the octant, alongside the sextant it helped lead to, had displaced earlier navigational instruments by 1780 and became a standard tool of maritime navigation.