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backstaff

noun

Definition of backstaff 

: an instrument similar to a cross-staff but fitted with a reflector and formerly used for taking the altitudes of heavenly bodies

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First Known Use of backstaff

1627, in the meaning defined above

History and Etymology for backstaff

back entry 1 + staff; from the position of the observer, whose back is turned toward the sun when using it

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The first known use of backstaff was in 1627

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