Cross-staff

A graduated navigational instrument for measuring the angular distance between celestial objects or between a celestial body and the horizon.

Transversal line: A graduated scale used to measure distances by proportional division. The cross-staff’s scale converts the crosspiece’s position into an angle.
Jacob's staff: A sighting instrument with a sliding crosspiece, used to measure angles in surveying and astronomy. This name was used for instruments closely related to the cross-staff.
Celestial navigation: Navigation that determines position or direction from observations of celestial bodies. Cross-staff sights helped navigators estimate their latitude.
Quadrant (instrument): An instrument with a graduated quarter-circle scale for measuring angles. Like the cross-staff, it measured celestial altitude but used a different sighting layout.
Similar triangles: Triangles with equal corresponding angles and proportional corresponding sides. The instrument’s angular scale relies on fixed geometric proportions.
Levi ben Gerson: A medieval Jewish philosopher, astronomer, and mathematician known in Latin as Gersonides. He described an instrument resembling the cross-staff in the fourteenth century.
Latitude: A geographic coordinate measuring angular distance north or south of the equator. The Sun’s altitude at local noon can be used to estimate latitude.
Sextant: A reflecting instrument that measures the angular distance between two visible objects. Its optical design made celestial measurements more practical aboard moving ships.
Parallax: The apparent shift of an object’s position when viewed from different locations. The observer aligns two sights to establish the measured angle.
Navigation in the Age of Discovery: The development of maritime navigation during European oceanic exploration from the fifteenth century onward. The cross-staff became a practical tool for measuring latitude at sea.

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Edmund GunterRelated: It belongs to the instrument-making tradition Gunter applied to surveying and navigation.