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
How it works
Diagram of a Kamal
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.
of a Cross-staff
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°
In addition, the smallest graduation on the staff is around 3°,
therefore it is impossible to use the Cross-staff in low latitude regions.