Tuesday, July 31, 2012

Jacob's Staff

Have you seen a modern surveyor using a single vertical rod with an instrument on top to measure property lines? That pole is nicknamed a "Jacob's Staff." Hundreds of years ago, however, the term "Jacob's Staff" was used to refer to more than one type of instrument; it's the other instrument that I want to discuss today.
A recreation of a Jacob's Staff.

The other instrument was also called a cross-staff, or a fore-staff, or (around the Mediterranean) a balestilha. It comprised a staff with cross-pieces designed to allow the user to determine angles of distant points. By aligning it with, say, Polaris (the North Star), a sailor could determine the angle between the star and the horizon. Matched against known measurements, the sailor then knew his latitude.

Knowledge of geometry and arithmetic also allowed the user to determine other measurements. If the height of a distant tower were known, that number and the angle known via the staff allowed you to determine your distance from the tower. Alternately, if you knew your distance from the base of a tall object, finding the angle from you to the top of it helped you determine how tall it was.

The Jacob's Staff had another name as well when used with heavenly bodies: radius astronomicus.

From an early book on navigation.
Where did it come from? The earliest description we have of the device is in the astronomical works of Gersonides (Levi ben Gerson, 1288-1344), who describes it as:
... a staff of 4.5 feet (1.4 m) long and about one inch (2.5 cm) wide, with six or seven perforated tablets which could slide along the staff, each tablet being an integral fraction of the staff length to facilitate calculation, used to measure the distance between stars or planets, and the altitudes and diameters of the Sun, Moon and stars. (Book of the Wars of the Lord)
Although some ascribe it to a contemporary of his, an astronomer named Jacob ben Machir (1236-1304). This might also explain its common name, since no other convincing etymology has come forth. The invention of the more precise sextant in the 18th century rendered the Jacob's Staff a quaint relic, and the name, when used, came to refer only to the simpler measuring stick we still see today.

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