Note: Do not rely on this information. It is very old.
Backstaff, an obsolete nautical instrument for taking the sun's altitude. It was so called because the observer, when taking his observation, turned his back to the sun. It was also called Davis's quadrant, from its inventor, John Davis, the navigator. The French called it the English quadrant. It superseded the more ancient Cross-staff, and consisted of two concentric circles, the arc of one radius being 60° and of the other 30°, with three vanes and the necessary frame. It was introduced about 1590, improved by Flamsteed, and generally superseded by Hadley's reflecting quadrant in 1731, though here and there it was in use up to the end of the last century.