Origin

In about 1594 John Davis, an English captain, developed a simple Back-staff which eliminated the problems encountered when using the Cross-staff. Davis' Back-staff was intended to be an improvement on the mariners' quadrants, astrolabes and cross-staves. The Davis Back-staff consisted 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.

How it works

The observer places the staff on his shoulder and stand 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.

Davis improved upon this design in a model with  two half-crosses, which divided an accurate scale into two parts, had the appearance of a large triangle equipped with a 30° arc at one end and a small 60° arc at the other. One scale was engraved on its upper side towards the front of the staff, the other was on its underside and at the back.

Remarks

Invention of the Back-staff eliminated the problems caused by using the Cross-staff, such as ocular parallax and damage to the eye due to looking at the Sun directly, because alignment is made on the shadow and the vane. In addition, the Back-staff can only be used to measure the altitude of the Sun and not other celestial bodies because only sunlight is strong enough to cast a shadow.