back staff

noun

an obsolete instrument for determining the altitude of the sun by facing away from the sun, sighting upon the horizon, adjusting a cursor until its shadow falls upon the sight through which the horizon appears, and measuring the resulting arc.

Origin of back staff

First recorded in 1620–30

Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

Examples from the Web for back staff

Historical Examples of back staff

This instrument was known also as "the back-staff" from the position of the observer with his back to the sun when using it.
Life Aboard a British Privateer in the Time of Queen Anne
Robert C. Leslie