Su Song's Astronomical Clock Tower in Kaifeng (1088-1092)

Su Song’s astronomical clock tower was a water-driven machine about 12 metres (roughly 40 feet) tall, built in Kaifeng, the capital of the Northern Song dynasty, between 1088 and 1092. Designed under the direction of the scholar-official Su Song (1020–1101) with the engineer Han Gonglian, it turned an armillary sphere and a star globe in time with the heavens and announced the hours with moving figures, bells and drums. Its water-wheel escapement and endless chain drive make it one of the landmarks in the history of clockmaking.

Background: why the Song needed a clock

In imperial China the calendar and the observation of the heavens were matters of state. The emperor’s legitimacy was linked to harmony between heaven and earth, and accurate calendars mattered for ritual, agriculture and diplomacy. Song dynasty China maintained official astronomers and observatories, and rival states, such as the Liao in the north, produced calendars of their own.

Chinese engineers had a long tradition of water-driven astronomical instruments. In the 2nd century AD Zhang Heng was credited with an armillary sphere turned by water power, and in 725 the monk Yi Xing and the engineer Liang Lingzan built a water-powered celestial globe with an early form of escapement. Su Song, a senior official with wide learning in medicine and astronomy, was ordered in 1086 to build a new armillary clock that would surpass these earlier devices.

Building the tower (1088–1092)

Su Song assembled a team led by Han Gonglian, a minor official skilled in mathematics and mechanics. They first built a small wooden working model, completed in 1088, and then a full-size wooden version. After testing, the main instruments were cast in bronze, and the complete machine was finished in 1092 in the palace grounds at Kaifeng.

Su Song then wrote a detailed illustrated account, the Xin Yixiang Fayao (New Design for an Armillary Clock), presented to the throne in the early 1090s. It contains dozens of drawings of the tower, its parts and its gearing, together with star maps. Because this book survived, historians know far more about Su Song’s clock than about any other early mechanical clock in the world.

How it worked

The tower had three main levels:

• Top: a bronze armillary sphere, an instrument of rings representing the celestial circles, used for sighting stars. A light removable roof made it one of the earliest observatories with an opening roof.
• Middle: a celestial globe inside a chamber, rotated so that its stars matched the real sky.
• Bottom: a pagoda-like front with five tiers of doors, where wooden figures appeared holding signs, ringing bells, striking gongs and beating drums to mark the hours and quarters of the day.

All of this was driven by a waterwheel, reported as about 3.4 metres (11 feet) across, with 36 scoops. Water from a constant-level tank poured into one scoop at a time. A balance mechanism, which Needham called the “celestial balance”, held the wheel still until the filling scoop became heavy enough to tip a lever. The wheel then moved forward by a single step and was locked again. This start-and-stop action is an escapement: it divides continuous power into equal, countable beats.

Power was carried up to the sphere at the top by a long vertical shaft and by a chain drive, the “celestial ladder”, which Su Song’s drawings show as an endless loop of linked metal pieces. It is the earliest known illustration of a power-transmitting chain drive.

Telling Chinese time

The figures in the tower followed the Chinese system of time, not the 24 equal hours used in Europe today. The day was divided into 12 “double hours” (shi), each named after one of the twelve earthly branches, and also into 100 equal units called ke, each a little under 15 minutes. Su Song’s mannequins displayed tablets for each double hour and for its beginning and middle, while others marked the ke with bells and drums. Night watches, whose length changed with the seasons, were also signalled.

Keeping the machine in step with the sky required the constant-level water tank to deliver a very even flow. Attendants refilled the upper reservoirs by hand, and the escapement turned the steady trickle of water into regular steps. The tower’s purpose was astronomical, so its accuracy was judged against the stars rather than against other clocks.

Su Song the statesman

Su Song was not a professional craftsman but a senior civil servant. He passed the jinshi examination in 1042 and served in many posts at court and in the provinces. In 1061 he completed an illustrated pharmacopoeia, the Bencao Tujing, describing medicinal plants, minerals and animals. In 1077 he travelled as an envoy to the Liao court, where, according to his biography, he noticed that the Liao and Song calendars disagreed about the date of the winter solstice, an embarrassment that helped convince the court of the need for better instruments.

His treatise also includes a set of star maps covering more than 1,400 stars, among the oldest printed star charts to survive. They show the sky in both cylindrical and polar views, a sophisticated choice of projection for the 11th century.

Key people

• Su Song (1020–1101): statesman, diplomat, astronomer and author of an illustrated pharmacopoeia; he briefly served as chancellor in 1092–1093.
• Han Gonglian: the mathematician and engineer who worked out the gearing and mechanism.
• Emperor Zhezong (reigned 1085–1100): the young emperor under whose reign the tower was completed.

Fate of the clock

The tower worked for little more than three decades. In 1127 the Jurchen Jin captured Kaifeng in the Jingkang Incident and carried off the imperial treasures. The clock was dismantled and taken north to the Jin capital, but it was never made to work there. In the south, the refugee Song court tried to rebuild it, reportedly with help from Su Song’s son, but the knowledge needed had been lost.

Myths and facts

Myth: Su Song built the first mechanical clock. Water-wheel escapements in China predate him by more than three centuries. His tower is, however, the most complete and best documented of them.

Myth: European clocks were copied from Su Song’s design. There is no evidence of a direct link. The weight-driven verge-and-foliot clocks of late 13th- and 14th-century Europe used a different mechanism, and their origins remain debated.

Fact: it was a scientific instrument as much as a timekeeper. Its main purpose was astronomical, keeping instruments aligned with the sky for observation and calendar work.

Legacy

Su Song’s book was rediscovered by modern historians, and in 1960 Joseph Needham, Wang Ling and Derek de Solla Price published Heavenly Clockwork, which brought the tower to wide international attention. Working reconstructions have since been built, including a full-size replica in Taichung, Taiwan, and the tower is now a symbol of Song technical achievement, along with the inventions described by Su Song’s contemporary Shen Kuo. Later astronomical clocks in Europe, such as the Prague Orloj, show a parallel fascination with clocks that model the heavens.

Timeline

• 725: Yi Xing and Liang Lingzan build a water-powered celestial globe with an escapement.
• 1086: Su Song is ordered to design a new armillary clock.
• 1088: A working wooden model is completed.
• 1090s, early: The bronze instruments are cast and installed.
• 1092: The full clock tower is completed in Kaifeng.
• c. 1092–1094: Su Song presents the illustrated Xin Yixiang Fayao.
• 1127: The Jin capture Kaifeng and carry the clock north.
• 1960: Heavenly Clockwork is published.

Why it matters

Su Song’s tower is the best-documented early clock with an escapement and contains the first known chain drive. It shows that Song China combined state-sponsored science, careful engineering and illustrated technical writing centuries before similar machines appeared elsewhere.