Celestial Navigation · Refraction — Low Altitude
Sights of bodies at very low altitudes (near the horizon) are avoided when possible because:
- AThe sextant cannot read low angles
- BThe body is too bright
- CRefraction is large and uncertain there, reducing accuracy✓ Correct
- DThe horizon is always invisible at low altitude
Explanation
Atmospheric refraction is greatest and most variable near the horizon (about 34' at 0°), and is sensitive to nonstandard temperature and pressure. Low-altitude sights therefore carry more error; navigators prefer bodies above roughly 15° when they can.
Authority: Nautical Almanac; Bowditch (Pub. No. 9)
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