Engraving of Hadley's quadrant and sextant, 1806. Sir Isaac Newton (1643-1727) invented the principle of the doubly reflecting navigation instrument (a reflecting quadrant), but never published it. Two men independently developed the octant around 1730: John Hadley (1682-1744), an English mathematician, and Thomas Godfrey (1704-1749), a glazier in Philadelphia. John Bird made the first sextant in 1757. The octant and later the sextant, replaced the Davis quadrant as the main instrument for navigation.