Observed altitude (Ho)

Observed altitude Ho is the fully corrected sextant altitude, the true altitude of a body’s center above the celestial horizon, ready to compare with the computed altitude Hc in the intercept method. It is built up from the sextant reading Hs in stages: apply index correction and dip to get apparent altitude, then the main correction for refraction, and for the Sun and Moon also semidiameter and parallax (the Moon needs the large parallax and augmentation terms). The Nautical Almanac altitude-correction tables supply each step. The intercept is Ho minus Hc, plotted toward the body when Ho is the greater.