Old instruments
Moon Sight
Direct answer
Moon Sight is a timed measurement of the Moon's limb relative to the horizon for celestial reduction.
Moon Sight is a lunar altitude observation used in historical astronomy, timekeeping, or navigation.
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At a glance
- Found in
- Historical Almanacs & Instruments
- Related
- Star Sight, Sun Sight, Planet Sight, Lunar Distance Method
Moon Sight is a lunar altitude observation used in historical astronomy, timekeeping, or navigation.
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The story behind Moon Sight
Moon sights supported position work and also the historical lunar-distance approach to time and longitude.
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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 Moon Sight matters
Moon Sight matters because the Moon extends observing opportunities but requires careful treatment of rapid motion, parallax, phase, and limb.
Moon Sight in practice: an example
A bright waxing Moon can provide a position line after sunset when its horizon remains visible.
Where readings of Moon Sight go wrong
Large horizontal parallax and uncertain limb illumination make shortcuts especially risky.
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
- Perpetual Calendar
- Running Fix
- Transit Instrument
- Armillary Sphere
- Calendar Wheel
- Cross Staff
- Greenwich Hour Angle
- Longitude by Chronometer
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Common questions
What is a moon sight?
Moon Sight is a timed measurement of the Moon's limb relative to the horizon for celestial reduction.
What do people get wrong about a moon sight?
Large horizontal parallax and uncertain limb illumination make shortcuts especially risky.
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