Davis Quadrant, Also Known As Backstaff, Made Of Wood C1730

A beautiful Davis quadrant in fruitwood, circa 1730. The backstaff is a navigational instrument used to measure the altitude of a celestial body, particularly the Sun or Moon. When observing the Sun, users held it behind their backs (hence the instrument's name) and observed the shadow cast by the upper weather vane on a horizontal weather vane. It was invented by the English navigator John Davis, who described it in his book *Seaman's Secrets* in 1594. The quadrant's arc was divided into two parts. The smaller radius arc, with a span of 60°, was mounted above the staff. The larger radius arc, with a span of 30°, was mounted below. The two arcs shared a common center. At this common center, a slotted horizontal weather vane was mounted. A movable weathervane was placed on the upper arch so that its shadow was cast on the horizontal weathervane. A movable weathervane was mounted on the lower arch. It is easier for a person to place a weathervane in a precise location than to read the arc from an arbitrary position. This is due to vernier acuity, the ability of a person to align two line segments accurately. Thus, a small-radius arc, marked with relatively few graduations, can be used to accurately place the weathervane in the shadow at a specific angle. On the other hand, moving the sighting weathervane to the point where the line to the horizon meets the shadow requires a large arc. The small-radius arc of 60 degrees has been restored. The Hammers are modern