Octant and sextant
Octante y sextante de reflexión
- Date
- 1731 (octante) / 1757 (sextante) · confidence high
- Place
- London, England / Philadelphia (Godfrey)
- Credited to
- John Hadley (octant, 1731) and Thomas Godfrey (independently); the sextant of John Campbell (1757)
- People
- 👤 John Hadley1682–1744👤 Thomas Godfrey1704–1749
- Field
- Movement
⚠️ Disputed: the sources disagree.
What it is
A double-reflection instrument with mirrors that brings the image of a celestial body down to the horizon, allowing altitudes and lunar distances to be measured to within minutes of arc from a moving ship. It replaced the cross-staff and the Davis quadrant.
Hadley presented two designs in 1731; Godfrey, a Philadelphia glazier, devised a similar one in 1730; Newton had described the principle in a manuscript of 1699 found after his death.
What it led to
- Lunar distances for longitude (the Nautical Almanac, 1767)
- Precise hydrographic surveys
- Standardised astronomical navigation until GPS
Built on
- Astronomical quadrantc. 850
- Cross-staffc. 1470-1520 (uso náutico)
- Mariner's astrolabec. 1480-1500
- Davis quadrant1594
- Vernier scale1631 (nonio de Nunes, 1542)
- Screw micrometer1638-1640
- Newton's proposal for a reflecting instrument (1699)
Made possible
- Lunar distance method and the Nautical Almanac1763-1767
- Sextant with artificial horizon (Gago Coutinho)1921
- Borda's reflecting circle
- Aeronautical bubble sextant