A ship's longitude cannot be measured accurately while at sea.

Latitude could be estimated from the height of the Sun or known stars, but longitude posed a harder problem. A ship needed to know how far east or west it had traveled from a reference meridian, and small navigation errors accumulated disastrously over long voyages.

The key was time. Earth rotates 360 degrees in 24 hours, so the difference between local solar time and the time at a reference meridian directly reveals longitude. The difficulty was carrying that reference time aboard a ship for weeks or months without a clock drifting under changes in temperature, humidity, and motion.

John Harrison spent decades developing marine timekeepers capable of doing exactly that. Trials of his designs showed that sufficiently accurate clocks could survive ocean voyages and make the time-difference method practical.

Astronomical lunar-distance methods also matured and competed with chronometers for a period, so the solution was not a one-instrument overnight revolution.

Modern radio time signals and satellite navigation make the old problem almost invisible. GPS still depends fundamentally on extremely precise time. Longitude became routine not because geography changed, but because portable timekeeping became accurate enough to turn Earth’s rotation into a measuring instrument.