When stars are observed, their light is considered to be a point source. This is not truly the case with the planets, but the disks are small enough that we can treat the light as being a point source, with Venus being a possible exception. However, the sun and the moon have large disks. The GP for the sun and moon listed in the Nautical Almanac is based on the center of the sun and moon. It is not practical to try to align the center of either the sun or moon with the horizon with the sextant. Instead, we align the upper or lower limb of the bodies with the horizon.
A correction for the body’s Semi Diameter must be made to account for the difference between the limb and the center of the body.
As the distances changes between the bodies throughout the year, the Semi Diameter for both the sun and the moon found on the daily pages of the Nautical Almanac also changes. The Semi Diameter of the sun and moon appears at the bottom of the daily pages of the Nautical Almanac.
When the apparent altitude with respect to the celestial horizon (Ha) is corrected for Atmospheric Refraction, and for Parallax and Semi Diameter (when appropriate), we arrive at the fully corrected observed altitude (Ho).