Celestial Navigation · Sextant Corrections — Refraction
Atmospheric refraction of a celestial body's light is greatest when the body is:
- ANear the zenith
- BAt low altitude near the horizon✓ Correct
- CExactly at 45 degrees
- DBelow the horizon
Explanation
Refraction bends light most when it passes obliquely through the most atmosphere — at low altitudes near the horizon — making the body appear higher than it is. The correction is always subtractive (negative).
Authority: The Nautical Almanac, altitude correction tables; Bowditch (Pub. No. 9)
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