In navigation the instrument is also called a cross-staff and was used to determine the vessel's latitude by measuring the altitude of Polaris or the sun.
The Jacob's staff, when used for astronomical observations, was also referred to as a radius astronomicus. With the demise of the cross-staff, in the modern era the name "Jacob's staff" is applied primarily to the device used to provide support for surveyor's instruments.
History
The origin of the name of the instrument is not certain. Some refer to the Biblical patriarch Jacob specifically Gen 32:11.[1] It may also take its name after its resemblance to Orion, referred to by the name of Jacob on some medieval star charts.[2][3] The name cross staff simply comes from its cruciform shape.
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