What major navigational challenge was solved by John Harrison's H4 marine chronometer in the 1760s?

Determining longitude at sea

Once the pendulum clock provided sufficient accuracy in stationary settings, the next major hurdle for timekeeping involved portability and stability at sea. Determining accurate longitude far from land was functionally impossible before highly accurate portable clocks existed because longitude relies entirely on precise time differences relative to a reference meridian. English clockmaker John Harrison dedicated decades to designing marine chronometers that could withstand the rocking and varying temperatures encountered on ocean voyages. His culmination, the H4 model developed in the 1760s, finally demonstrated the requisite accuracy, proving dependable enough to solve the longitude problem and enabling reliable, accurate location tracking across vast expanses of ocean.