The octant is a navigational instrument that was developed around 1730 almost simultaneously by an English mathematician, John Hadley, and an American glazier, Thomas Godfrey.

What did the octant replace?

The octant survived through the end of the eighteenth century but it was eventually replaced by the sextant which could measure larger angles, up to 120 degrees.

What did John Hadley invent?

In 1730, independently of Thomas Godfrey of Philadelphia, Hadley invented a quadrant (actually a double-reflecting octant) for measuring the altitude of the Sun or a star above the horizon to find geographic position at sea. His double-reflecting principle made accurate determinations of location much easier.