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John Harrison · Interactive explainer
Longitude by chronometer explorer
Use the classic Harrison-era idea: when local apparent noon happens on the ship, what Greenwich time does the chronometer show? The page turns that into longitude, then shows how clock drift becomes east-west position error.
What this page answers
Many history pages tell the John Harrison story without making the working method concrete. This page answers the practical follow-up: if the ship sees local noon and the Greenwich chronometer reads a different time, what longitude does that imply?
- Targets search intent around longitude by chronometer, the longitude problem, and how Harrison’s clocks were used.
- Makes the east-versus-west sign explicit instead of leaving it buried in prose.
- Shows why a drift rate that sounds tiny can become a real navigational miss on a long voyage.
How the sign works
At your local apparent noon, the Sun is highest where you are. If the Greenwich chronometer already reads later than 12:00, Greenwich has reached noon earlier than you, so you are west of Greenwich. If it still reads earlier than 12:00, you are east of Greenwich.
Why Harrison mattered
The longitude problem was not just about knowing a formula. Navigators already knew that time difference implied longitude. The hard part was carrying trustworthy Greenwich time across an ocean without the clock wandering too far.