Description: The cross-staff, or Jacob's Staff, is device for measuring the angle between objects. Similar to the sextant, octant, and quadrant, the cross-staff can be used to measure the location of objects in the sky, or to determine the distance to objects on the horizon.
Unlike the sextant, the cross-staff has no mirrors or filters to help one dim the light of the sun. It is also very awkward to use on a rocking boat. To help fix some of these issues, the cross-staff was developed into the "backstaff" by John Davis in 1594. This improved instrument allowed one to take measurements of the sun's position using shadows, and without having to look directly into the sun.
Usage Dates: 1300s - 1700s
Claudius Ptolemaeus holding a cross-staff. This image is rather anachronistic: the cross-staff was not invented until many centuries after Ptolemy's death.
Woodcut showing a master and apprentice practicing with cross-staffs.
Close inspection of the left-center of this image will reveal a ship's navigator using a cross-staff to the left, and another using an astrolabe to the right.
A simple wooden cross-staff at Harvard's Museum of Historical Scientific Instruments.
A close-up on the same cross-staff. Numbers indicating the angle can be seen.
The Cross-staff and its predecessor the Kamal, from the Institute of Navigation's Navigation Museum
The Cross-Staff at the National University of Singapore
How to make your own cross-staff at "From Stargazers to Starships"
Wikipedia on the Jacob's Staff
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