The officers and captain navigated the ship by the sun and stars, using various instruments to work out the position of the ship or the time, and a lead to check the depth of water to avoid running aground. The other essentials were a compass and a chronometer.

The backstaff was used to determine the height of the sun above the horizon. The user turned their back to the sun and looked through a vane that moved around the large arc, and lined up with the horizon the shadow cast by a vane that mopved around the small arc. The angles on the arcs were added together to give the sun's altitude.

This example was made by H. Gregory of London, and unfortunately the vanes are missing.