Passage V

The Flow of Time

Nine hundred years ago, Emperor Zhezong of China, ordered the design and construction of a clock built to keep time more accurately than other clocks. This would be no simple timepiece and because Chinese dynasties continued to astrology, they relied on complicated clocks that not only kept time but also helped track stars, planets, the sun, and the moon. An eminent scientist and bureaucrat named Su Song lead Zhezong’s ambitious project.

Using his expertise in calendrical science, Su Song created a spectacular timepiece housed within an ornate forty-foot-tall tower. At the tower’s top sat an armillary sphere, or a nest of metal rings representing celestial reference points such as the horizon and the sun’s path—that rotated in sync with the earth, enabling precise astronomical observations. Inside the tower, a sphere depicting the sky revolved to display the stars that were overhead. Besides, below the star sphere, the tower’s open sides exposed a detailed model of a five-story pagoda. Automated figurines would appear in the pagoda’s doorways and ring bells to announce hours, sunsets, seasons, and other chronological events.

The clock’s inner workings were equally remarkable. Hidden in the tower, a waterwheel eleven feet in diameter powered the entire clock. Therefore, water would pour at a constant rate into one of the wheel’s thirty-six buckets. When the bucket was full, the water’s weight pulled it down, rotating the waterwheel. Then a stop mechanism halted the wheel and positioned the next bucket for filling. Chinese clockmakers had long used waterwheels, but Su Song’s stop mechanism, which regulated the inertia of the waterwheel, represented significant innovation.

Unfortunately, after Su Song’s clock ran for thirty years, invaders stole it. Later the clock vanished altogether. It would be a few hundred years until with the refinement of mechanical clocks in Europe other clocks approached the complexity of Su Song’s masterpiece.