Celestial Navigation · Sextant Corrections
Atmospheric refraction in an altitude observation is:
- AAlways subtracted, and greatest at low altitudes near the horizon✓ Correct
- BAlways added, and greatest near the zenith
- CZero at all altitudes
- DSubtracted only for the Moon
Explanation
Refraction bends light so a body appears higher than it truly is, so the correction is always subtracted. It is greatest near the horizon (about 34' at 0° altitude) and negligible near the zenith.
Authority: Bowditch (Pub. No. 9)
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