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Jacob’s staff or cross-staff is a stick with length markings to make qualitative judgements of the height and angle of an object relative to the user of the staff, used for astronomical observation and navigation

Perpignan, 1344: The Ralbag passed away in Perpignan, Catalan (Perpinyà, France).

R’ Levi ben Gershon, known by his acronym Ralbag, and in the conventional world as Gersonides, is known in the Jewish world, through his commentary on Tanach.

In the astronomical world, he’s accredited with the invention of the Jacob’s Staff. In Wikipedia, it’s explained as follows: “The simplest use of a Jacob’s staff is to make qualitative judgments of the height and angle of an object relative to the user of the staff.” The Jacob’s Staff was used by sailors before the sextant was invented.

Source: Sinai vol. 47, page 74

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