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Hadley's Quadrant

John Hadley, an English mathematician and Fellow of the Royal Society of London invented a reflecting quadrant for navigation in 1730. The instrument could measure angles up to 90° in an arc of only 45° and so was known by two names—Hadley’s quadrant or an octant.

This quadrant is a prototype tested at sea by George Hadley, brother of John, and is the earliest example known to survive.

A major use of the Hadley’s quadrant was to measure the angular elevation of the North Star or the Sun above the horizon in order to find one’s latitude. The system of mirrors made this measurement easier on the rolling seas than earlier navigational instruments for the same purpose.

Curator Sara Schechner demonstrates the use of a quadrant.

CHSI Inv. Number 1998-1-1663
Date: 1735
Jonathan Sisson
London, England


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