How could a clock help a ship find its longitude?

Far from land, a ship’s clock could help answer a geographical question: how far east or west had the ship traveled? The important clock was a marine chronometer, built to keep a known reference time during a voyage. Its reading could be compared with local time determined from observations of the sky.

Earth turns through 360 degrees in about 24 hours relative to the mean Sun. That connects one hour of time difference with 15 degrees of longitude. Places farther east reach the same stage of the solar day earlier. The clock does not measure distance across the sea directly; it preserves the comparison needed to infer an angle around Earth.

In our simplified example, local solar noon occurs when the reference clock reads 14:00. The reference place reached noon two hours earlier, so the ship is 30 degrees west of it. A reference reading of 10:00 at the ship’s noon would instead place the ship 30 degrees east. A four-minute timing error corresponds to one degree of longitude.

Real navigation required corrections: apparent solar time differs seasonally from mean solar time, and observations and clocks have errors. The picture sets those corrections aside to isolate the relation. Reliable timekeepers, including John Harrison’s sea clocks, made this comparison practical at sea alongside other navigational methods.