quadrant

nautical instrument

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Last updated

July 13, 2026

Quick Summary

A quadrant is a traditional nautical instrument used for navigation, shaped like a quarter of a circle. It was employed by sailors to calculate altitude, which aided in determining their position at sea.1

Historically, sailors used instruments like the quadrant and astrolabe to measure the elevation angle of celestial bodies, such as the polestar or the Sun at noon, to determine latitude. These measurements were crucial for navigating across oceans and establishing trade routes. While the quadrant was an early tool, it was eventually succeeded by more advanced instruments like the cross-staff, octant, and sextant, which offered greater accuracy.2

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John Hadley
…of Philadelphia, Hadley invented a quadrant (actually a double-reflecting octant) for measuring the altitude of the Sun or a star above the horizon to find geographic position at sea. His double-reflecting principle made accurate determinations of location much easier. Hadley also fixed a spirit level to the instrument so that…
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…of Philadelphia, Hadley invented a quadrant (actually a double-reflecting octant) for measuring the altitude of the Sun or a star above the horizon to find geographic position at sea. His double-reflecting principle made accurate determinations of location much easier. Hadley also fixed a spirit level to the instrument so that…
Read More

navigation
Latitude measurements
…seem to have been the quadrant and the astrolabe, long known to astronomers. For both devices the reference direction was actually the vertical, rather than the horizontal, but conversion of the readings was an elementary matter. The mariner’s astrolabe, however, was less widely used than its 16th-century successor, the cross-staff,…
Read More
…seem to have been the quadrant and the astrolabe, long known to astronomers. For both devices the reference direction was actually the vertical, rather than the horizontal, but conversion of the readings was an elementary matter. The mariner’s astrolabe, however, was less widely used than its 16th-century successor, the cross-staff,…
Read More