By Michael Brooks We have seven days a week because the Babylonians named one for each planet they knew. There are 12 hours in each day and each night because they gave one hour to each sign of the zodiac. Sixty minutes in an hour because they thought 60 an auspicious number. Surely we can do better. We decimalised currency: why can’t we decimalise time? Divide the day into 10 “hours”, with 100 “minutes” in each hour and 100 “seconds” in each minute. Suddenly, it’s much easier to work out exactly how much time you have on your hands. (That’s literally true: since we have 10 fingers and thumbs, the decimal system feels more natural.) China was working with a decimal system millennia ago. They gave it up when Jesuit astronomers brought them the European duodecimal system, with its twelfths. Decimal time made a brief reappearance after the 1789 French revolution. Mathematician Pierre-Simon Laplace even had a watch that gave decimal time, and he wrote his celebrated Treatise on Celestial Mechanics using decimal time units. Historical efforts to reintroduce decimal time have failed largely due to economics. Powerful nations, which relied on trade carried out by ships, used navigation systems – sextants, theodolites, charts and tables – that were configured for the Babylonian system. These days, with GPS, that obstacle has evaporated. GPS uses atomic time units, defined by the frequency of radiation emitted when caesium-133 undergoes a particular atomic transition. Decimalised time could easily be defined by the same method. “If we were starting from scratch today, we would still have a definition of the second based on some atomic transition: