Celestial Navigation · Sextant Corrections — Order
Apparent altitude (Ha), the value used to enter the main altitude correction table, is the sextant altitude corrected for:
- AIndex error and dip✓ Correct
- BRefraction and parallax
- CSemidiameter only
- DDeclination
Explanation
Sextant altitude (hs) is corrected for index error and dip to give apparent altitude (ha). The main correction (refraction, plus semidiameter/parallax as applicable) is then applied to ha to obtain observed altitude (Ho).
Authority: The Nautical Almanac, altitude correction procedure
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