Celestial Navigation · Sextant Corrections — Order of Application
The correct order of corrections from sextant altitude (hs) to observed altitude (Ho) is:
- AApply semidiameter, then index error, then dip
- BApply refraction first, then dip, then index error
- CApply index error and dip to get apparent altitude (ha), then the main correction (refraction, semidiameter, parallax)✓ Correct
- DApply all corrections in any order; the result is the same
Explanation
hs is first corrected for index error and dip to give the apparent altitude (ha). The main altitude correction — which combines refraction, semidiameter, and parallax — is then taken from the tables using ha, yielding the observed altitude Ho. Dip cannot be applied after refraction because the tables are entered with ha.
Authority: Nautical Almanac; Bowditch (Pub. No. 9)
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