Old instruments
Semidiameter Correction
Direct answer
Semidiameter Correction is the angular adjustment from an observed limb of the Sun or Moon to the body's center.
Semidiameter Correction is a visible-limb center correction used in historical astronomy, timekeeping, or navigation.
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At a glance
- Found in
- Historical Almanacs & Instruments
- Related
- Dip Correction, Refraction Correction, Almanac, Nautical Almanac
Semidiameter Correction is a visible-limb center correction used in historical astronomy, timekeeping, or navigation.
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The story behind Semidiameter Correction
Semidiameter columns and combined correction tables became standard nautical-almanac aids for solar and lunar sights.
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Why Semidiameter Correction matters
Semidiameter Correction matters because sextants often measure a bright body's edge while the almanac calculation refers to its center.
Semidiameter Correction in practice: an example
A lower-limb Sun observation is shifted from the lower edge to the center using the listed semidiameter.
Where readings of Semidiameter Correction go wrong
Upper and lower limb require opposite treatment, and the Moon's apparent semidiameter changes appreciably with distance.
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
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Common questions
What is Semidiameter Correction?
Semidiameter Correction is the angular adjustment from an observed limb of the Sun or Moon to the body's center.
What do people get wrong about Semidiameter Correction?
Upper and lower limb require opposite treatment, and the Moon's apparent semidiameter changes appreciably with distance.
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