Jacob’s staff

A graduated rod fitted with a sliding crosspiece, used to measure angles and estimate distances in surveying and astronomy.

Cross-staff: A sighting instrument with a graduated staff and a movable crosspiece for measuring angular separation. This common name emphasizes the staff-and-transom arrangement used in the same measuring principle.
Surveying: The measurement and mapping of land boundaries, elevations, and positions. Surveyors used the staff to estimate distances and angles in the field.
Levi ben Gerson: A fourteenth-century Jewish philosopher, mathematician, and astronomer who designed scientific instruments. He described and illustrated an instrument often identified with the Jacob’s staff.
Quadrant (instrument): A quarter-circle instrument used to measure angles, often including celestial altitude. Unlike the staff, a quadrant reads an angle from a graduated arc.
Transom (instrument): The crosspiece that slides along a measuring staff to set a sighting angle. Moving the transom changes the angle subtended between the observer’s sight lines.
Celestial navigation: The determination of position at sea from observations of celestial bodies. Related cross-staff instruments measured the altitude of the Sun or stars.
Jacob ben Makir: A thirteenth-century Jewish astronomer and mathematician active in Provence. The instrument’s name has sometimes been associated with this astronomer, though its origin is uncertain.
Sextant: An optical instrument that measures the angle between two visible objects using a graduated arc. The sextant replaced simple cross-staff sighting for many navigational measurements.
Similar triangles: Triangles with equal corresponding angles and proportional corresponding side lengths. The instrument’s geometry relates staff positions to sighting angles through triangle proportions.
Altitude (astronomy): The angular height of a celestial object above the horizon. Astronomers used the instrument to measure a body's elevation in the sky.

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