Celestial Navigation · Moon — Corrections
Why does a moon sight require both a main altitude correction and an additional (HP) correction from the Nautical Almanac?
- AMoon sights ignore dip
- BThe moon has no semidiameter to correct
- CThe moon is too dim for a single correction
- DThe moon's large, variable horizontal parallax means an extra correction based on HP is needed beyond the standard refraction/SD correction✓ Correct
Explanation
Because the moon is close to Earth, its horizontal parallax (HP) is large and changes noticeably, and its semidiameter varies. The almanac's moon tables use a two-part correction — a main altitude correction plus an HP-based correction — to reduce the sight accurately.
Authority: Nautical Almanac, moon altitude correction tables
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