Old instruments
Sextant
Direct answer
Sextant: A Sextant is a double-reflection instrument used to measure the angular separation between two visible objects, most famously a celestial body and the horizon for navigation.
A Sextant measures an angle, often between a celestial body and the horizon.
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At a glance
- Found in
- Historical Almanacs & Instruments
- Related
- Almanac, Astronomical Almanac, Nautical Almanac, Astrolabe
A Sextant measures an angle, often between a celestial body and the horizon.
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The story behind Sextant
Eighteenth-century reflecting instruments developed into the marine sextant, whose stable optical geometry became central to practical celestial navigation.
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Why Sextant matters
Combined with exact time, almanac data, and corrections, a sextant observation can help determine a navigator's line of position without satellite navigation.
Sextant in practice: an example
A navigator brings the reflected lower limb of the Sun to the sea horizon, records the angle and UTC, then reduces the sight with almanac data.
Where readings of Sextant go wrong
A common error is to look at the Sun without the instrument's proper shades and training, and never rely on an unverified sight as the sole safety-critical position source.
Sources and editorial method
Research references used for this entry include:
- Royal Museums Greenwich: Sextant - Museum documentation of a marine sextant used for solar and lunar observations and navigation.
- U.S. Naval Observatory: Astronomical and Navigational Almanacs - Official description of current Astronomical Almanac and Nautical Almanac publications.
- Source
- The Great Almanac Dictionary
- Category
- Historical Almanacs & Instruments
- Last updated
- 2026-09-26
Follow a sextant sight from angle to position
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Common questions
What is a Sextant?
A Sextant is a double-reflection instrument used to measure the angular separation between two visible objects, most famously a celestial body and the horizon for navigation.
What do people get wrong about a Sextant?
A common error is to look at the Sun without the instrument's proper shades and training, and never rely on an unverified sight as the sole safety-critical position source.
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