Backstaff

A navigational instrument that measures the Sun’s altitude by letting an observer face away from it and align its shadow with the horizon.

Solar altitude: The angular height of the Sun above the horizon. The backstaff’s reading is the Sun’s altitude above the visible horizon.
John Davis: An English navigator and explorer who developed the backstaff in the late sixteenth century. Davis introduced the instrument as a safer way to measure the Sun’s altitude.
Celestial navigation: Navigation that determines position from observations of celestial bodies. The backstaff is a specialized instrument within this broader navigational method.
Davis quadrant: A large backstaff variant with graduated arcs used to measure the Sun’s altitude. This prominent form made the backstaff’s shadow-sighting principle more precise and widely usable.
Umbra: The fully shaded region behind an opaque object illuminated by a light source. The instrument uses a vane’s shadow as a safe proxy for sighting the Sun.
Cross-staff: A graduated rod with a sliding crosspiece used to measure the angle between two sights. The backstaff grew from the cross-staff tradition but replaced direct solar sighting with shadow alignment.
Latitude: A geographic coordinate measuring angular distance north or south of the equator. Solar altitude at local noon could be converted into latitude with date corrections.
Octant: A reflecting navigation instrument that measures angles up to about 90 degrees. The octant offered compact, direct angular measurement and helped supersede shadow instruments.
Horizon: The apparent boundary separating Earth or sea from the sky. The observer aligns the shadow with the horizon to establish the reference line.
Navigation Act 1563: An English statute that required pilots and masters to demonstrate navigational competence. The law formed part of the Tudor-era demand for more dependable navigational practice.

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Jacob’s staffCompared with: It avoided looking directly at the Sun, a hazard of cross-staff observation.