Astronomer Royal, who tested and later disputed Harrison’s claims
Plate 050
1735–1761
England
No photograph is shown for this exhibition. The museum displays an image only where it can show the object honestly.
About this date
John Harrison completed H1 in 1735 and H4 in 1759. H4 was tested at sea in 1761–62.
About credit
Harrison built the first chronometer accurate enough to solve the longitude problem. Other methods, such as lunar distances, competed with it.
Chapter II
The world before
Life before the Marine Chronometer
Sailors could find latitude but not longitude. Ships were lost when they misjudged where they were.
II
The human achievement
How it came about
The Longitude Act of 1714 offered a large prize. Harrison built a series of clocks. H4 resembled a large pocket watch and lost only a few seconds over a transatlantic crossing. Parliament awarded him a sum in 1773 after a long dispute.
III
Understanding the invention
How it works
The Marine Chronometer, explained in two steps.
01
Step 01 of 02
Time is longitude
The Earth turns 15 degrees per hour, so a clock set to home time and local noon reveal how far east or west you are.
Time is longitude
Compensation
IV
The transformation
What changed for humanity
Seafaring
Navigation became far safer.
Mapmaking
Accurate longitudes improved charts.
Knowing the time at a reference port reveals the difference in longitude. Accurate position at sea saved lives and made trade safer.
V
Before and after
Life before and after The Marine Chronometer
Before
Latitude known; longitude guessed.
After
Longitude within a few miles.
On larger screens, drag the divider (or use the arrow keys) to give more room to the world before or after.