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.

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.