Old instruments
Pelorus
Direct answer
Pelorus is a sighting compass-card device used to measure directions relative to a vessel's heading.
Pelorus is a relative-bearing instrument used in historical astronomy, timekeeping, or navigation.
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At a glance
- Found in
- Historical Almanacs & Instruments
- Related
- Compass Rose, Magnetic Compass, Celestial Navigation, Dead Reckoning
Pelorus is a relative-bearing 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 Pelorus
Pelorus instruments became standard bridge aids for bearing observations and compass checks.
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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 Pelorus matters
Pelorus matters because relative bearings help identify landmarks, check compass error, and track other vessels without relying on a magnetic needle.
Pelorus in practice: an example
A navigator can take the relative bearing of the Sun and combine it with heading to compare observed and computed azimuth.
Where readings of Pelorus go wrong
A misaligned lubber line or changing heading makes the bearing unreliable.
Sources and editorial method
Research references used for this entry include:
- 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
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More in Historical Almanacs & Instruments
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Common questions
What is Pelorus?
Pelorus is a sighting compass-card device used to measure directions relative to a vessel's heading.
What do people get wrong about Pelorus?
A misaligned lubber line or changing heading makes the bearing unreliable.
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