How a clock found longitude at sea: Harrison's H4 and the £20,000 prize
30seconds
In 1707 a British fleet hit the rocks off Scilly because navigators could not find longitude. Seven years later Parliament offered £20,000 for a method accurate to half a degree.
The key is that the Sun moves 15° of longitude per hour, so a clock that still keeps Greenwich time turns a noon sighting into a position. Drag a ship across the Atlantic, then find how much clock drift over a six-week voyage the prize allows, and see why John Harrison's H4, five seconds slow at Jamaica, was so remarkable.
Deep dive · 4 min
More in History
View all →
How a map of dots found a cholera pump in 1854: John Snow and Broad Street
In late August 1854, cholera tore through a few streets of Soho, killing roughly 600 people in weeks. Almost everyone blamed foul air. John Snow suspected the…
How a beam with no date gets one: tree rings and crossdating
A beam from a ruined house has no date on it, but its rings do. Trees from the same region share the same wide and thin years, so a pattern of ring widths can…
How a stick and a well measured the whole Earth: Eratosthenes
Around 240 BC, the librarian Eratosthenes measured the size of the Earth using a well, a stick and a shadow. At noon on the summer solstice the sun shone…
Why washing hands saved mothers in 1847: Semmelweis and the two wards
In 1846, mothers in one Vienna maternity ward died at four times the rate of mothers in the ward next door. The doctors' ward was losing 11.4% of its patients…
How a 2,000-year-old bronze machine predicted eclipses with the number 223
In 1901, divers pulled a corroded bronze machine from a shipwreck off the Greek island of Antikythera. One dial ended in the number 223, written in Greek…
How one doctor's map traced a cholera outbreak to a single pump
In 1854 most doctors blamed cholera on bad air. When it tore through Soho, killing hundreds in days, John Snow went door to door asking one question: where did…