Emperor Tenji Introduces Water Clock: Time Measured in Ōtsu

Emperor Tenji installed a water clock called a Rokoku at the Omi Palace in Otsu on June 10, 671 AD, standardizing timekeeping across the Japanese imperial court for the first time. The instrument measured the passage of time by regulating the flow of water between a series of vessels, with markers indicating the twelve two-hour periods that divided the Japanese day. Drums and bells announced each period, imposing a synchronized schedule on a court that had previously operated on the imprecise rhythms of sunrise and sunset. The Rokoku was modeled on Chinese clepsydrae that had been in use for centuries. Japan’s adoption of the technology was part of a broader wave of institutional borrowing from Tang dynasty China that reshaped Japanese governance, writing, religion, and urban planning during the seventh century. Tenji, who had been a driving force behind the Taika Reforms of 645 that restructured the Japanese state along Chinese lines, understood that centralized administration required centralized time. Tax collection, court ceremonies, military mobilization, and diplomatic audiences all depended on officials agreeing on when things happened. Water clocks had inherent limitations. Their accuracy depended on maintaining a constant flow rate, which varied with temperature, water quality, and the gradual accumulation of mineral deposits in the vessels. In winter, the water could freeze. Skilled technicians were required to maintain and recalibrate the instruments regularly. Despite these constraints, the Rokoku served as the primary timekeeping device for the Japanese court for centuries, supplemented eventually by incense clocks that measured time through the predictable burning rate of calibrated sticks. June 10 is celebrated in Japan as Time Day (Toki no Kinenbi), a national observance established in 1920 to promote punctuality and respect for time. The holiday traces directly to Tenji’s water clock installation thirteen centuries earlier. The emperor’s decision to measure time precisely reflected a truth about governance that transcends cultures: power belongs to those who define the schedule.