A ship sits somewhere on the ocean at a position the simulation knows but you don't. Pick one of ten real bright navigational stars — Polaris, Sirius, Vega, Arcturus, Capella and others — placed in the night sky at their true altitude and azimuth as seen from the ship, and sight it with the virtual sextant. Each sight compares the measured altitude (Ho) against the altitude computed for your dead-reckoning assumed position (Hc) using the actual intercept-method formulas from celestial navigation: LHA from the star's SHA and Greenwich hour angle, then the altitude-declination-latitude identity. The difference plots as a line of position on the chart alongside; two or three LOPs from different stars cross to give a fix, and the sextant-error slider controls how tightly they converge on the ship's true position.