Old instruments
Gnomon
Direct answer
Gnomon is the part of a sundial or solar instrument whose shadow or light boundary indicates solar position.
Gnomon is a shadow-casting solar indicator used in historical astronomy, timekeeping, or navigation.
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At a glance
- Found in
- Historical Almanacs & Instruments
- Related
- Alidade, Plumb Line, Meridian Circle, Transit Instrument
Gnomon is a shadow-casting solar indicator 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 Gnomon
Gnomons range from simple vertical posts to polar-aligned edges on precision dials and meridian lines.
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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 Gnomon matters
Gnomon matters because its shadow makes Earth's rotation and the Sun's seasonal path directly observable.
Gnomon in practice: an example
A polar-style gnomon casts a shadow across hour lines arranged for the site latitude.
Where readings of Gnomon go wrong
A decorative pointer is not necessarily aligned as a functional gnomon, and its shadow gives solar rather than civil time.
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
Explore Gnomon
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Common questions
What is a gnomon?
Gnomon is the part of a sundial or solar instrument whose shadow or light boundary indicates solar position.
What do people get wrong about a gnomon?
A decorative pointer is not necessarily aligned as a functional gnomon, and its shadow gives solar rather than civil time.
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