Huygens' Pendulum Clock (1656): The Revolution in Timekeeping

The pendulum clock was invented by the Dutch scientist Christiaan Huygens in The Hague in December 1656. By letting a swinging pendulum regulate a clock’s gears, he made timekeeping dozens of times more accurate than before. The pendulum clock remained the world’s most precise timekeeper for almost three centuries and transformed science, navigation and daily life.

Background: clocks before the pendulum

Mechanical clocks had existed in Europe since the late thirteenth century. They were driven by weights or springs and regulated by a verge escapement with a swinging bar called a foliot. Even good examples typically drifted by around 15 minutes a day and had to be reset often using sundials.

Galileo had noticed that a pendulum’s swings take nearly the same time whether they are wide or narrow, a property called isochronism. In 1641, late in his life, he described how a pendulum might control a clock, and his son Vincenzo began a model, but no working clock resulted.

Christiaan Huygens

Christiaan Huygens was born in The Hague on 14 April 1629, the son of Constantijn Huygens, a diplomat and poet who served the princes of Orange. He studied law and mathematics at Leiden and Breda, and by his twenties was among the leading mathematicians in Europe. With his brother Constantijn he ground his own telescope lenses.

Astronomy gave him a practical reason to want better clocks, since precise observations depended on knowing the exact time. Astronomers measured the positions of stars partly by timing when they crossed a fixed line in the sky, and an error of a minute meant an error of a quarter of a degree.

Huygens also had a mathematician’s interest in the problem. The pendulum offered a natural, repeatable rhythm, and he saw that a clock could be built around it rather than merely checked against it. He worked at the same time on probability, collisions of bodies and the shape of curves, all of which fed into his later theory of the pendulum.

The invention

Huygens designed his pendulum clock at the end of 1656, around Christmas. The pendulum, linked to the clock’s escapement, swung at a steady rate and released the gear train in regular steps, while the clock’s weights kept the pendulum swinging.

On 16 June 1657 the States General of the Dutch Republic granted him a patent. He licensed the design to the Hague clockmaker Salomon Coster, who began building clocks the same year. A spring-driven Coster clock signed and dated 1657 survives in Museum Boerhaave in Leiden. In 1658 Huygens published a short description, Horologium.

The results were dramatic. Early pendulum clocks kept time to within about a minute a day, and with improvements to around 10–15 seconds. For the first time clocks could usefully show minutes, and soon seconds.

The design also spread quickly. Coster’s patent applied only in the Dutch Republic, and clockmakers in France and England soon built their own versions; John Fromanteel, who had worked with Coster in The Hague, brought the design to London, where the Fromanteel family advertised pendulum clocks in 1658. Many older clocks were converted to the new system by having a pendulum added, and in some church towers the original verge-and-foliot mechanisms were rebuilt as pendulum clocks.

The new accuracy changed expectations. Clock faces began to carry minute hands as standard, and astronomers, surveyors and physicians could time events with a precision that had been impossible a few years earlier. Huygens himself used the pendulum to obtain an accurate value for the acceleration due to gravity.

A priority dispute followed. Supporters of Galileo in Florence argued that the idea was his, and Viviani later wrote an account of Galileo’s design. Historians accept that Galileo conceived of a pendulum clock, but Huygens’ invention was independent, and his was the first that actually worked.

Horologium Oscillatorium and the cycloid

Huygens knew that a simple pendulum is not perfectly isochronous: wide swings take slightly longer than narrow ones. He proved mathematically that a bob moving along a curve called a cycloid would swing in exactly the same time whatever the amplitude. To approximate this he fitted curved metal “cheeks” on either side of the suspension cord.

He set out this work in Horologium Oscillatorium, published in Paris in 1673. Beyond clocks, it contained a theory of the centre of oscillation, a formula for the period of a pendulum, and results on curves and on centrifugal force. Historians regard it as one of the most important works of mechanics before Isaac Newton’s Principia.

Further developments

Around 1670 English clockmakers introduced the anchor escapement, credited to Robert Hooke or William Clement. It allowed small swings and a long pendulum about a metre in length beating once a second, which led to the tall longcase or “grandfather” clock.

Huygens hoped his clocks would solve the problem of finding longitude at sea, which required carrying accurate time. Pendulum clocks were tried on voyages in the 1660s, but the motion of ships disturbed them. In 1675 he designed a watch regulated by a spiral balance spring, which led to a bitter priority dispute with Hooke. The sea clock was finally achieved by John Harrison in the eighteenth century.

Myths and facts

Myth: Galileo invented the pendulum clock. Galileo discovered the near-regular swing of the pendulum and sketched a clock design, but it was never completed in his lifetime. Huygens built and patented the first working pendulum clock.

Myth: the pendulum clock solved navigation at sea. Pendulums could not cope with a ship’s rolling and pitching. Accurate timekeeping at sea came only with spring-driven chronometers in the eighteenth century.

Myth: a pendulum always keeps perfect time. Even a pendulum’s period changes slightly with the size of its swing and with temperature, which lengthens or shortens the rod. Huygens tackled the first problem with his cycloidal cheeks; later clockmakers solved the second with compensated pendulums.

Huygens’ other work

Huygens’ interests ranged across science. Using his own telescopes, he discovered Saturn’s largest moon, Titan, on 25 March 1655, and in Systema Saturnium (1659) he explained that Saturn is surrounded by a thin flat ring. In 1657 he published one of the first books on probability theory.

His wave theory of light, worked out in the 1670s and published as Traité de la lumière in 1690, explained reflection and refraction through a principle still taught as Huygens’ principle. He was elected a Fellow of the Royal Society in 1663 and was a founding member of the French Académie des Sciences in 1666, living in Paris for many years. He died in The Hague on 8 July 1695.

Timeline

• 1629, 14 April: Christiaan Huygens is born in The Hague.
• 1641: Galileo describes an idea for a pendulum-regulated clock.
• 1655, 25 March: Huygens discovers Titan.
• 1656, December: Huygens invents the pendulum clock.
• 1657, 16 June: The States General grants his patent; Salomon Coster builds the first clocks.
• 1658: Huygens publishes Horologium.
• c. 1670: The anchor escapement is introduced in England.
• 1673: Horologium Oscillatorium is published in Paris.
• 1695, 8 July: Huygens dies in The Hague.

Why it matters

The pendulum clock made precise time measurement possible for the first time. It supported advances in astronomy and physics, set new standards for everyday timekeeping, and remained the most accurate type of clock until quartz clocks appeared in the twentieth century.