How Long Is An Hour?

There are science fiction stories that describe other worlds, stars and galaxies where inhabitants measure time in a different way than us. But you don’t have to read science fiction for this. History shows that people in the past already used different time systems than we use today. For example, in 1792, during the French Revolution, the authorities officially introduced decimal time, which divided the day into 10 hour. It also divided each hour into 100 minutes and each minute into 1oo seconds, as opposed to our standard time, which divides the day into 24 hours, splits each hour into 60 minutes and each minute into 60 seconds. The length of one decimal hour converted into our standard time is two hours and 24 minutes. Mind you, and quite astonishingly, decimal time was also used in China, as early as the first millennium BCE. The ancient Chinese employed clepsydras, or waterclocks, to divide the day into 100 units, which they called “ke”. Actually, alongside with the decimal system, the Chinese also used a dozenal time keeping system that divided the midnight- to- midnight day into 12 double hours. And during the Han Dynasty, the philosopher Huan Tan, who flourished about two thousand years ago, wrote that he examined clepsydrae and sundials in order to compare their accuracy. About a thousand years later, in 976, the astronomer Zhang Sixun replaced water with mercury to solve the problem of freezing water in clepsydrae in cold wheather. Chinese engineers in this epoch constructed marvelously sophisticated mechanisms, such as Su Song’s astronomical clock tower. Su Song was an 11th century polymath, whose astronomical clock tower comprised of a clepsydra tank, waterwheel, pendulum escapement wheel gear, chain drive to power an armillary sphere and 113 striking clock jacks to sound the hours on a gong.