Old instruments
Octant
Direct answer
Octant is a reflecting instrument whose forty-five-degree arc can measure angles up to about ninety degrees.
Octant is a double-reflection angle instrument used in historical astronomy, timekeeping, or navigation.
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At a glance
- Found in
- Historical Almanacs & Instruments
- Related
- Backstaff, Davis Quadrant, Reflecting Circle, Hadley Quadrant
Octant is a double-reflection angle instrument used in historical astronomy, timekeeping, or navigation.
An engraved functional orientation; the page explains the real instrument, method, or publication.
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The story behind Octant
Eighteenth-century octants helped replace backstaffs and directly preceded the wider-range sextant.
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The Book of the Sky
How people learned to read the sky, from Babylon to the day you were born.
Why Octant matters
Octant matters because double reflection keeps two sight lines in view while an index arm measures their angular separation.
Octant in practice: an example
An observer can bring a reflected star down to the sea horizon without moving the direct horizon view.
Where readings of Octant go wrong
Index error, mirror alignment, horizon quality, and unsafe solar viewing all require correction or control.
Sources and editorial method
Research references used for this entry include:
- Royal Museums Greenwich: Astronomical and Navigational Instruments - Museum reference collection for historical observing, calculating, timekeeping, and navigation instruments.
- National Geospatial-Intelligence Agency: The American Practical Navigator - Official modern reference for navigation principles, celestial observations, sight reduction, and navigational tables.
- Source
- The Great Almanac Dictionary
- Category
- Historical Almanacs & Instruments
- Last updated
- 2026-09-26
Explore Octant
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More in Historical Almanacs & Instruments
- Refraction Correction
- Star Catalogue
- Alidade
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- Clepsydra
- Equation of Time Table
- Latitude by Meridian Altitude
- Mariner's Astrolabe
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Common questions
What is an octant?
Octant is a reflecting instrument whose forty-five-degree arc can measure angles up to about ninety degrees.
Why does an octant matter?
Octant matters because double reflection keeps two sight lines in view while an index arm measures their angular separation.
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