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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… History4 min read17 Oct 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… History5 min read14 Oct 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… History4 min read11 Oct 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… History3 min read7 Oct 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… History4 min read4 Oct 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… History5 min read1 Oct How a 9th-century scholar cracked secret codes by counting letters For roughly nine centuries after Julius Caesar, swapping each letter for another was a perfectly good way to keep a secret. Then, in 9th-century Baghdad, the… History5 min read30 Sep Why October 1582 lost ten days: a calendar year 11 minutes too long In 1582, Thursday 4 October was followed by Friday 15 October. Ten dates were simply deleted across Catholic Europe. The reason was an error of about 11… History5 min read28 Sep How Eratosthenes measured the whole Earth with a stick and a shadow Around 240 BC, Eratosthenes, head of the Library of Alexandria, measured the size of the whole Earth with a stick and a shadow. At noon on midsummer, sticks in… History4 min read27 Sep How sailors found longitude before satellites: a race against the clock Latitude is easy: measure the height of the Sun or the Pole Star. Longitude is hard, because the Earth turns one degree every four minutes. To know how far… History6 min read26 Sep