Octant

Octants are instruments used to observe the altitude of celestial bodies above the horizon, allowing a ship's latitude to be determined. They replaced earlier instruments designed for the same purpose, called "Davis quadrants." Octants were first presented at the Royal Society in London by John Hadley in 1731.

The octant is based on the laws of light reflection. It was a breakthrough for this type of measurement as it removed the inconvenience of having to line up the horizon with one eye and celestial body with the other. It comprises a 45° arc with two limiting radii and a circular pin with a hole in the center which acts as a lens, a small glass mirror, and a large mirror.

This model belonged to Lieutenant General in the Spanish Navy (Armada) Antonio de Ulloa (1716–95) and was made around 1734.