Old instruments
Sight Reduction
Direct answer
Sight Reduction is the calculation that turns a timed celestial altitude into a line of position or related navigation result.
Sight Reduction is a celestial-position calculation used in historical astronomy, timekeeping, or navigation.
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At a glance
- Found in
- Historical Almanacs & Instruments
- Related
- Planet Sight, Lunar Distance Method, Intercept Method, Marcq Saint-Hilaire Method
Sight Reduction is a celestial-position calculation used in historical astronomy, timekeeping, or navigation.
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The story behind Sight Reduction
Sight-reduction methods evolved from long trigonometric calculations to compact tables, plotting methods, calculators, and software.
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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 Sight Reduction matters
Sight Reduction matters because a raw sextant angle becomes geographically meaningful only after corrections and comparison with a computed body direction.
Sight Reduction in practice: an example
Comparing corrected and computed altitude gives an intercept to plot toward or away from the body azimuth.
Where readings of Sight Reduction go wrong
One sign, time, date, or assumed-position error can yield a neat-looking but wrong line.
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.
- U.S. Naval Observatory: Astronomical and Navigational Almanacs - Official descriptions of the Astronomical Almanac, Nautical Almanac, and their tabular data.
- Source
- The Great Almanac Dictionary
- Category
- Historical Almanacs & Instruments
- Last updated
- 2026-09-26
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More in Historical Almanacs & Instruments
- Water Clock
- Astrolabe
- Celestial Position Fix
- Dioptra
- Heliometer
- Magnetic Compass
- Noon Sight
- Precession Table
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Common questions
What is Sight Reduction?
Sight Reduction is the calculation that turns a timed celestial altitude into a line of position or related navigation result.
What do people get wrong about Sight Reduction?
One sign, time, date, or assumed-position error can yield a neat-looking but wrong line.
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