ABSTRACTA proof is given that the effects of the height of the eye, earth curvature, and atmospheric refraction can be safely ignored when vertical sextant angle measurements short of the horizon are used in practical navigation problems. The expressionDtan(hs) =H, relating the distance‐off, sextant altitude, and navigation aid height, is shown to provide satisfactory accuracy for safe navigation where dip short of the horizon techniques are applicable. The double table entry procedure suggested in Bowditch for vertical sextant angle calculations is shown to be unnecessary and is incorrect with respect to the prediction of the effects of both earth curvature and atmospheric refractio