Total correction tables.
The altitude correction tables for the sun are given at the beginning of the Nautical Almanac. They cover all altitudes from 0° to 90° and the heights of eye from 1 to 48 metres. The tables provide corrections to apply to both the upper and lower limbs of the sun and are in two parts.

The first part contains the effects of refraction, semi-diameter and parallax whilst the second part provides the corrections for dip. Since the sun’s distance from the earth varies throughout the year, the tables make allowance for the change in semi-diameter.

The tables are divided into two sections, one based upon a semi-diameter of 16.15’ which is used during the period October to March and one based upon a semi-diameter of 15.9’ which is used during April to September. The first table is entered with the apparent altitude and the second with the observer’s height of eye. All corrections in the first part are applied positively to the apparent altitude when the lower limb is observed and negatively if the sun’s upper limb is observed.

True altitude.
This is the resultant altitude after all the above corrections have been applied. It is the altitude measured above the celestial horizon from the centre of the earth.