Marine Glossary

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additional altitude correction

Used only in the sight reduction of the moon, Mars, or Venus (the three closest celestial bodies), this correction accounts for the parallax of their light rays - that is, since each of these bodies is so close to the earth, the light ray we see it with is not strictly parallel to the light ray from it that passes through its geographical position (GP). The theory of celestial navigation assumes that the distance along the earth's surface between the observer and the GP is equal to the zenith distance (z) of the body, but this is only true if these two light rays are parallel. So for these three close bodies this extra correction is required. See Parallax.

Another, completely different, type of Additional Altitude Correction is also discussed and tabulated in the Table A4 of the Nautical Almanac. These are additional corrections for non-standard refraction in unusual atmospheric conditions. In principle, these corrections apply to all sights, but from a practical point of view they can be neglected. The corrections are very small for all sights except those near the horizon (which should be avoided anyway if possible). There is no space for this type of additional correction on the Starpath Workforms. It is recommended that this atmospheric correction just be ignored, but bear in mind that any sights within some 5° of the horizon will be uncertain by plus or minus 5' or so. See Refraction.