The octant, also called the ‘Hadley quadrant’, was invented by John Hadley in 1730. They have a 45° arc but can measure 90°. They operate similarly to the back-staff, but use a mirror to reflect the sun through colored filters to line up the horizon. Like the reflecting circle and later sextant light reflected from an object is displaced by twice that angle, so even though octants were fitted with an arc of only one-eighth of a circle, they gave an effective reading of twice that or 45°, same as an quadrant. So the ancient quadrant lived on. Octants were made of wood, often mahogany and ebony in the late 1700s with brass and ivory fittings and silvered scales. As testimony to their enduring value less elegant models were still being certified for marine navigation as late as 1925. The sextant was conceptualised in the late 1750s based on the contemporary invention of the reflecting circle even though it resembles an octant. The earlier ones were large, heavy and cumbersome. They employ a relatively small 60-degree arc and refined methods of marking degrees on the scale lead to hand-held versions that became rapidly popular by the end of the century. The sextant with its superior accuracy allowed crude estimates of longitude based on the ‘lunar distance’ method (distance of certain stars from the moon) that was discovered in 1767. The American military adapted sextants to aircraft in 1922. They were designed for reference to the broad and steep angle between the aircraft and horizon. Some were attached to gyroscopes and called gyro-sextants. In 1933 an electrically driven gyro-octant was tested. Sextants are still made and sold as backup navigation devises, and seamen are still instructed in their use.