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.
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Origin of back staff
First recorded in 1620–30
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
Examples from the Web for 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