The spreadsheet alt_corr.xls performs the corrections to the sextant altitude Hs (cell B1) that are needed to produce the apparent altitude Ha (cells B6, B7, B8) and the observed altitude Ho (cells B12, B13, B14). The index correction goes to cell B2. Cell B4 contains a yes/no (Y/N) answer to the question whether a reflecting artificial horizon was used. The semidiameter correction is entered in cell B9; this is positive for lower limb and negative for upper limb observations. The hight of eye in cell E1 (enter “ft” for feet or “m” for meters in cell F1) determines the dip correction. Cells E2 and E3 control the refraction correction; the standard values are Temperature = 10 oC and Pressure = 1010 mb. Cell E6 contains the value of the horizontal parallax (HP) in arc minutes. The Moon parallax can also be corrected for the oblateness of the Earth by entering the latitude (E8) and azimuth (E9). The semidiameter value from either cell E11 (Sun - typical preset value, or from the almanac) or E12 (Moon - computed from the HP) is to be copied (with the appropriate sign characterizing the limb) into cell B11.

Summary for spreadsheet alt_corr.xls:

Input cells: B1, B2, B4, B11, E1, F1, E2, E3, (E6, E8, E9 optional)

Output cells: B6, B7, B8, B12, B13, B14

Input/Output cells: E11, E12

The preset example contained in the spreadsheet is the upper limb Moon sight from p. 281 of the Nautical Almanac, 2009 Commercial Edition.