Celestial Navigation · Sextant Corrections — Refraction
Atmospheric refraction makes a body appear HIGHER than it really is, so the refraction correction is:
- ASubtracted, and is greatest at low altitudes near the horizon✓ Correct
- BAdded, and is greatest overhead
- CZero at the horizon
- DConstant at all altitudes
Explanation
Refraction bends light downward, raising the apparent altitude, so the correction is subtracted. It is largest near the horizon (about 34' at 0° altitude) and decreases to near zero at the zenith. This is why low-altitude sights are less reliable.
Authority: Nautical Almanac, Altitude Correction Tables
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