H4 (Harrison's fourth chronometer)

H4, or Harrison's fourth chronometer, is a highly accurate marine timekeeping device developed by John Harrison in the 18th century to solve navigation problems at sea. It maintains precise time regardless of ship movements and environmental conditions, enabling sailors to determine their longitude accurately. H4 is a large, portable, period-appropriate clock with a metal case, featuring innovations that minimized effects of motion, temperature, and humidity on its accuracy. Its reliability revolutionized navigation, significantly improving maritime safety and exploration during its time.