When observing sun or moon with a marine sextant or theodolite, it is not possible to locate the center of the body with sufficient accuracy.
It is therefore common practice to measure the altitude of the upper or lower limb of the body and add or subtract the apparent semi diameter, SD, the angular distance of the respective limb from the center.
We correct for the geocentric SD, the SD measured by a fictitious observer at the center the earth, since the celestial horizon and the center of the earth. The geocentric semi diameters of sun and moon are given on the daily pages of the Nautical Almanac. We can also calculate the geocentric SD of the moon from the tabulated horizontal parallax.