Celestial Navigation · Sextant Corrections — Semidiameter
When observing the Sun's lower limb, the semidiameter correction is:
- AAdditive (added to bring to center)✓ Correct
- BSubtractive
- CNever applied
- DEqual to the dip
Explanation
Measuring the lower limb means the center is about one semidiameter higher, so the correction is added. Observing the upper limb makes the semidiameter correction subtractive. The almanac's main Sun correction combines refraction, semidiameter, and parallax.
Authority: The Nautical Almanac, Sun altitude correction; Bowditch (Pub. No. 9)
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