Celestial Navigation · Altitude Corrections — Nonstandard Conditions
The additional altitude correction for nonstandard temperature and pressure is most important when observing bodies that are:
- ABelow the horizon
- BNear the zenith
- CExactly at 45° altitude
- DAt low altitudes near the horizon✓ Correct
Explanation
Refraction — and therefore its sensitivity to air temperature and barometric pressure — is greatest at low altitudes. The Almanac's additional-corrections table is applied for sights well below about 10° when conditions differ markedly from the standard 10°C and 1010 mb.
Authority: Nautical Almanac, Additional Corrections table
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