Instruments

Sextant

Citation Formats

General Reference

Science Atlas. "Sextant." Accessed August 30, 2026. https://science.interactiveatlas.org/instruments/sextant.

APA Style

Science Atlas. (n.d.). Sextant. Retrieved August 30, 2026, from https://science.interactiveatlas.org/instruments/sextant

BibTeX

@misc{scienceatlas-sextant,
  author = {Science Atlas},
  title = {Sextant},
  year = {2026},
  url = {https://science.interactiveatlas.org/instruments/sextant},
  note = {Accessed August 30, 2026}
}

A sextant is a doubly reflecting navigation instrument that measures the angular distance between two visible objects, most often a celestial body and the horizon, by bringing the reflected image of one into alignment with the direct view of the other in a pair of mirrors. The instrument's principle was first implemented around 1731, independently, by the English mathematician John Hadley and the American glazier Thomas Godfrey; both built working reflecting octants using the same double reflection principle Isaac Newton had described but never published decades earlier. Enlarged to a sixty degree arc reading up to one hundred and twenty degrees, the design became the standard tool of celestial navigation at sea for over two centuries.

Facts

Invented Yearthe subject's own account

1731 1

Measuresthe subject's own account

The angular distance between two visible objects, most commonly a celestial body and the horizon, used to fix a position by celestial navigation. 1

Operating Principlethe subject's own account

An index mirror rotated on a graduated arc brings the reflected image of one object into alignment, in a half silvered horizon glass, with the direct view of a second object; the angle read off the arc at that alignment is the angular distance between them. 1

Operating Rangethe subject's own account

Graduated to read angles up to about 120 degrees, twice the sixty degree arc that gives the instrument its name. 1

Open Questions

Resolution

No single figure; typically on the order of a tenth of a minute of arc on a marine sextant, depending on the vernier or micrometer drum.

Not stated as a single fixed figure in the cited source; practical reading precision depends on the instrument's vernier or micrometer drum and the observer's skill, commonly on the order of a tenth of a minute of arc on a marine sextant, rather than one specified value.

Cross-Tradition Connections