Intercept method (Marc St. Hilaire)

The intercept method, set out by Marcq Saint-Hilaire in 1875, is the standard way to reduce a celestial sight to a line of position. The navigator picks an assumed position, computes the altitude and azimuth a body should have there (Hc and Zn), then compares Hc with the sextant altitude after corrections (Ho). The intercept is Ho minus Hc, plotted in nautical miles from the assumed position toward the body if Ho is greater, away if less (the rule: greater toward, GOAT). The line of position is drawn perpendicular to the azimuth at the intercept point; two or more crossed lines give a celestial fix.