The Slide Rule Put Men on the Moon — Then Vanished in 18 Months

For 350 years, the slide rule was the quiet tool behind the modern world — from bridges and skyscrapers to the Saturn V rocket that carried men to the Moon. Engineers trusted it because it didn’t just calculate; it forced them to understand the problem. Then, in barely 18 months, a small calculator with a glowing red display made centuries of mastery vanish almost overnight.