backstaff

nautical instrument

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Last updated

June 18, 2026

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The backstaff was a nautical instrument used for measuring the altitude of celestial bodies, primarily for navigation. It was an improvement upon the earlier cross-staff, as it allowed the observer to take measurements without looking directly at the sun, thus reducing the risk of eye damage.12

Invented by English navigator John Davis in 1594, the backstaff featured a reflector and was used with the observer's back turned towards the sun. This design made it more practical and safer for taking solar altitudes compared to its predecessor. The backstaff remained in common use until the 18th century, eventually being superseded by the octant and sextant.12

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navigation
Latitude measurements
His instrument, called the backstaff because it was used with the observer’s back to the Sun, remained common even after 1731 when the octant (an early form of the modern sextant) was demonstrated independently by John Hadley of England (see photograph) and Thomas Godfrey of Philadelphia.
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His instrument, called the backstaff because it was used with the observer’s back to the Sun, remained common even after 1731 when the octant (an early form of the modern sextant) was demonstrated independently by John Hadley of England (see photograph) and Thomas Godfrey of Philadelphia.
Read More