Celestial Navigation · Sextant Corrections — Semidiameter
When observing the sun's lower limb, how is the semidiameter handled to reduce to the center of the sun?
- ASemidiameter is added (it is built into the lower-limb altitude correction)✓ Correct
- BSemidiameter is subtracted for the lower limb
- CSemidiameter is ignored for the sun
- DSemidiameter is applied only at meridian passage
Explanation
Sights are reduced to the body's center. Observing the lower limb means the altitude is short by one semidiameter, so it is added (the almanac's lower-limb correction includes refraction, semidiameter, and parallax). For an upper-limb sun sight the semidiameter is subtracted.
Authority: Nautical Almanac, sun altitude corrections; Bowditch (Pub. No. 9)
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