The need to measure the height of the stars on the horizon, to determine the latitude and from it the position at sea, has been essential since ancient times.
The oldest and less accurate instruments were the astrolabe and the balestriglia (cross staff), with direct vision only. The latter, also called Jacob's scale, was a perfectly squared parallelepiped stick, 70 cm long, with a cross of four rods proportionate in length to the four sides of the stick.
From the balestriglia derived the Davis's quadrant made by the English John Davis (1550-1605), the same who, between 1585 and 1587, made a polar expedition in search for a northwest passage to America, discovering on that occasion the Strait which bears his name, between Greenland and Baffin Island. In 1594 he published a work entitled The Seaman's Secret, where he describes the instrument he invented. This one was more effective in allowing a more accurate measurement of the height of the stars.
The quadrant is called back-staff because it is used with the sun on the back, to avoid being dazzled when measuring its height on the horizon. It consists of a graduated staff, a half-cross in the shape of an arc of a circle on the radius of the staff with a fixed vane, and a brass horizon vane with a slit in it at the fore-end of the staff.
The observer places the staff on his shoulder and stands with his back to the sun. With the horizon vane lined up with the horizon, he slides the half-cross back and forth until the shadow of its vane falls across the slit in the bottom vane while the horizon remains visible through the slit. By doing this, the observer is able to sight both the sun and the horizon while his back is towards the sun.