Celestial Navigation · Sextant Corrections — Index Error
Your sextant has an index error of 2.5' 'on the arc.' How is it applied to the sextant altitude?
- AIgnored if under 3.0'
- BAdded (+2.5'); errors on the arc are added
- CSubtracted (−2.5'); errors on the arc are subtracted, off the arc are added✓ Correct
- DApplied only to star sights
Explanation
Index error is the instrument error read against the arc. The mnemonic: 'if it's on, take it off; if it's off, put it on.' An error on the arc is subtracted; an error off the arc is added. Here 2.5' on the arc is subtracted.
Authority: Bowditch (Pub. No. 9), index error; Nautical Almanac
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