John Harrison solved the problem of longitude at sea — one of the greatest technical challenges of the 18th century — with the H4 marine chronometer, whose error on the trial voyage to Jamaica was only 5.1 seconds over 81 days. With it, ocean navigation gained an extremely precise time base that transformed the safety and reliability of transoceanic routes. Harrison was a self-taught clockmaker who competed for decades against the Greenwich astronomical establishment, which favored lunar methods, and took decades to receive official recognition.