Origins

The Cross-staff, or Jacob’s staff, is believed to be invented around 400 BC by the Chaldeans but was used mainly by the astronomers. Only in 1514, Johann Werner proposed to use it for navigation.

How it works

Diagram of a Kamal

The invention of the Cross-staff was inspired by the Kamal used by Arab navigators, which was also known to the Portuguese as the 'Balestilha of the moors'. The Kamal consisted of a length of cord attached to the center of a rectangular plaque of wood with knots on the cord that correspond to the latitude of a particular port. The observer will hold onto the appropriate knot to his eye and stretched the plaque so that he could see the horizon in line with its lower edge. The upper edge will determine if the observer is in the right latitude.

Similarly, a Cross-staff (below) was made up of a long staff, usually 36 inches long, and four shorter sliding cross pieces of varying length, that can moves up and down the longer staff. The four sides of the staff are all graduated differently in degree to suit each cross piece. The observer will choose an appropriate cross piece and thread it onto the long staff. By holding the Cross-staff to the eye, the observer will shift the cross piece until its upper and lower edges is in line with the celestials body and the horizon respectively. The altitude of the celestial body can then be read off from the appropriate scale. Both the Kamal and the Cross-staff can only be used to find altitude of a celestial body.

Diagram of a Cross-staff

Remarks

The Cross-staff is a much simpler device than the Astrolabe, but its reading is still affected by many other factors. For example, looking at two points at the same time can cause ocular parallax, and the need to look directly at the blinding sun when taking its meridian altitude (This problem was partially solved by using a smoked glass). The staff must also be positioned correctly on the cheekbone, so that the eye would be on the terminal point in the axis in order to obtain an accurate reading.

There is also some restriction to the usage of the Cross-staff. Due to the restriction of the angles between two objects that the human eyes can see at the same time, the angle that can be measured by the Cross-staff is restricted from around 20° to 60°. In addition, the smallest graduation on the staff is around 3°, therefore it is impossible to use the Cross-staff in low latitude regions.