Old instruments
Plumb Line
Direct answer
Plumb Line is a suspended weight and cord that establishes the local vertical.
Plumb Line is a gravity-reference instrument used in historical astronomy, timekeeping, or navigation.
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At a glance
- Found in
- Historical Almanacs & Instruments
- Related
- Dioptra, Alidade, Gnomon, Meridian Circle
Plumb Line is a gravity-reference 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 Plumb Line
Plumb lines accompanied quadrants, building tools, surveying frames, and astronomical instruments for centuries.
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The Book of the Sky
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Why Plumb Line matters
Plumb Line matters because a simple gravity direction lets many angle instruments reference altitude and level.
Plumb Line in practice: an example
A quadrant held in the target direction can use its plumb line to mark altitude on the graduated arc.
Where readings of Plumb Line go wrong
Wind, ship motion, contact with the frame, and local acceleration prevent a steady true vertical.
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.
- Source
- The Great Almanac Dictionary
- Category
- Historical Almanacs & Instruments
- Last updated
- 2026-09-26
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More in Historical Almanacs & Instruments
- Sextant
- Volvelle
- Astrolabe
- Celestial Navigation
- Declination Table
- Hadley Quadrant
- Magnetic Compass
- Nocturnal Instrument
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Common questions
What is a plumb line?
Plumb Line is a suspended weight and cord that establishes the local vertical.
What do people get wrong about a plumb line?
Wind, ship motion, contact with the frame, and local acceleration prevent a steady true vertical.
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