It is usually easier for an observer to find either the upper or lower limb of the sun rather than its center. Lewis, when using the octant, always shot the sun's lower limb. The angular difference between the sun's center and its upper or lower limb is called its 'semidiameter' (half diameter). The Nautical Almanac Lewis used had a table giving the sun's semidiameter at Greenwich noon for the 1st, 7th, 13th, 19th and 25th of each month. For July 25, 1805 it was 15'46.7" and on August 1 it was 15'47.5" For his observation on July 29, Lewis determines a semidiameter of 15'47". He adds this to the result of Step E to find the true altitude's lower limb.
