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Hadley's Quadrant: Detail

This detail from the upper hinge area shows (right to left, A) the pinhole sight, B) the large mirror on the movable index arm, C) the green glass shade, and D) the smaller half-silvered horizon mirror. The user looks through the sight at the horizon through clear glass portion of the horizon mirror.

By positioning the central index arm, the image of a star is reflected by the large mirror down to the silvered half of horizon mirror, and from it back to the eye. When the star’s image is aligned with the horizon, the angle between them is read off the divided scale on the lower limb of the quadrant.

The green glass shade can be interposed between the mirrors like a sunglass to protect the user’s eye under bright conditions.

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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