Ho is the observed altitude of the star once corrections have been applied. For our example, the star is Capella. The sextant reading is 29° 46.0' at time 08:00:00 UT (or GMT) on 17 July 2018. Height of eye = 10 ft. Index correction = -0.4'. Chief Sheedy uses the information to determine observed altitude, Ho.
The sight reduction applies each of the corrections to the sextant altitude (Hs) to obtain the observed altitude (Ho). The first correction is the index correction, unique to the sextant being used.
The second correction depends on the observer’s elevation above sea level, and is called the dip correction because the higher we are, the lower—or more dipped—the distant horizon appears. The dip correction accounts for this visual difference. It is in a pre-computed table in the inside front cover of The Nautical Almanac. Lastly, we apply altitude corrections, which are essentially corrections due to Earth's atmosphere bending light rays. These are in pre-computed tables in The Nautical Almanac. For the Moon, Venus, Mars, or if the temperature is extreme, we’ll apply additional corrections.
Watch and listen as Chief Sheedy demonstrates the sight reduction process for an 0800 UT sight of the star Capella.