Sight Reduction

Sight reduction is the mathematical process of converting a celestial observation—a measured sextant altitude and GMT—into a position line that can be plotted on the navigational chart. The process involves extracting the body's Greenwich Hour Angle (GHA) and declination from the Nautical Almanac, computing the Local Hour Angle (LHA) using the assumed or DR longitude, and then entering sight reduction tables—such as HO 229, NP 401 (AP3270), or equivalent calculator programs—to find the calculated altitude (Hc) and azimuth (Z). The difference between the observed altitude (Ho) and calculated altitude (Hc) is the intercept, plotted toward or away from the azimuth from the assumed position. Bowditch (American Practical Navigator) covers the complete procedure. Each sight reduction yields a single position line; a minimum of two simultaneous or near-simultaneous position lines crossing at a good angle constitutes a celestial fix. The OOW or second officer performing sight reductions must work methodically, checking each step against known values and plotting the position line carefully to avoid accumulated errors. STCW Table A-II/2 specifies celestial sight reduction as a competency for masters and chief mates, and regular practice on passage maintains the skill at a usable standard.