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The Pendulum Clock

A swinging weight turned clocks from rough guides into precision instruments. Here is how Galileo's idea became Huygens's invention.

By David Ray · clocks.fyi

Galileo and the swinging lamp

The pendulum story usually begins in Pisa Cathedral around 1583, with a young Galileo watching a lamp swing. The Museo Galileo in Florence explains that this account comes from his follower Vincenzo Viviani, who wrote it in a life of Galileo in 1654, roughly 70 years after the event.

Historian Antonio Favaro later called the tale a legend, because the lamp visitors see in the cathedral today was hung there after Galileo had left Pisa. More recent research, the museum notes, found that a different votive lamp did hang in the cathedral in Galileo's day, so the story may hold a grain of truth. That older lamp is now in Pisa's Monumental Cemetery.

What is certain is that Galileo studied pendulums. NIST says he was investigating their motion as early as 1582, and the Museo Galileo credits him with discovering isochronism: pendulums of equal length take the same time to swing, whether the swing is wide or narrow.

A clock Galileo never saw

Galileo had a practical reason to want a better clock. In a letter of June 1637 to Laurens Reael, he proposed finding longitude at sea by timing the moons of Jupiter, a method that needed precise timekeeping, and suggested a clock based on the pendulum.

In Viviani's account, Galileo worked out in 1641 how a pendulum could regulate weight-driven and spring clocks. He was blind by then and could not draw plans or make models, so when his son Vincenzio came one day from Florence to visit him at Arcetri, he explained the idea to him.

Vincenzio wanted to build it himself, so that no craftsman could leak the design before it was presented to the Grand Duke of Tuscany and to the Dutch States General as a way to find longitude. After his father's death he lost his enthusiasm, and according to Viviani he did not start making the clock until April 1649.

NIST notes that Galileo sketched a pendulum clock but died in 1642 without building one. A 19th-century copy of the original drawing by Viviani and Vincenzio survives in the Museo Galileo, which also holds models based on the design.

Want the exact time right now? See it on a big, clean clock at OnlineClock.app, free, with no sign-up.

Huygens makes it work

The Dutch scientist Christiaan Huygens made the first working pendulum clock in 1656, according to NIST. Guinness World Records gives a slightly fuller sequence: Huygens conceived the design late in 1656, and the Hague clockmaker Salomon Coster turned it into a working clock in June 1657. That split between designer and maker explains why both years appear in histories of the invention.

Guinness lists that June 1657 clock as the first operational pendulum clock, and says the oldest known pendulum clock by Coster, dated 1657, is now in the Rijksmuseum Boerhaave in Leiden. Huygens set out his design the following year in his book Horologium, published in 1658.

Credit was fought over. Huygens claimed the invention in 1658, and Viviani vigorously challenged him on Galileo's behalf, according to the Museo Galileo. Viviani's account of Galileo's design was a letter to Prince Leopold de' Medici dated 20 August 1659. Guinness notes that Huygens did credit Galileo for recognizing that a pendulum's steady swing suits timekeeping, even as he claimed the clock as his own.

Why a pendulum keeps better time

Before the pendulum, clocks were regulated by a verge and foliot, a weighted bar pushed back and forth by the escape wheel, or by a balance wheel, and neither had a strong natural rhythm of its own. A pendulum is different. NIST describes Huygens's clock as regulated by a mechanism with a natural period of oscillation.

That natural period comes mostly from the pendulum's length. The clock's job becomes simply to give the pendulum a small push each swing and count the swings, so small changes in the power reaching it matter much less.

The improvement was dramatic. NIST says Huygens's early clock had an error of less than a minute a day, and his later refinements brought that below 10 seconds a day. This is Galileo's isochronism put to work: because a pendulum of a given length keeps a steady beat, the rest of the clock only has to follow it.

See also: a free online metronome for keeping a steady beat on OnlineClock.app.

A wave of conversions

The new technology spread fast. John Robey, writing in the Antiquarian Horological Society's journal, cites 1658 as the accepted date for the first pendulum clocks made in London.

Owners of older clocks wanted the improvement too. Many English lantern clocks were converted from a balance wheel to a verge escapement with a short pendulum, and later to an anchor escapement with a long one. The unusual lantern clock Robey studied went through exactly those stages. He notes that, because of such conversions, few lantern clocks survive with their original balance.

Not every clockmaker switched at once. Robey points out that balance-wheel clocks were still being made in the English provinces into the 1680s, decades after the pendulum appeared.

Three centuries of refinement

Clockmakers kept pushing. Guinness World Records says that by the end of the 17th century, good pendulum clocks kept time to about a second a day, which made it worthwhile to add the first seconds hands.

NIST places that milestone a little later, crediting George Graham with reaching one second a day in 1721. Graham did it by compensating for the way a pendulum's length changes with temperature. That matters because length sets the pendulum's beat: a rod that stretches slightly in warm weather swings a little more slowly, and the clock falls behind.

Precision kept climbing. In 1889 Siegmund Riefler's clock, with a nearly free pendulum, reached about a hundredth of a second a day. The Shortt clock, demonstrated in 1921, used two pendulums, a master and a slave, and became one of the most famous precision clocks, according to NIST.

The pendulum never solved Galileo's original problem, though. Huygens hoped his clocks could find longitude at sea, but Guinness World Records notes that his designs were too delicate for a moving ship. Accurate marine timekeeping came with John Harrison's chronometers from the 1740s.

By then a new rival was coming. From the 1920s, NIST notes, quartz crystal clocks offered timekeeping with no gears or escapement to disturb their rhythm, beginning the next chapter of precision.

See also: the tall pendulum clocks known as grandfather clocks on OnlineClock.app.

Questions people ask

Who invented the pendulum clock?

Christiaan Huygens designed the first working pendulum clock in 1656, and Salomon Coster built it in 1657. Galileo had earlier studied pendulums and worked out a pendulum clock design, but never built it.

Did Galileo really watch a lamp swing in Pisa Cathedral?

The story comes from his follower Viviani, writing decades later. The lamp in the cathedral today was hung after Galileo's time, but another lamp did hang there then, so the tale may be partly true.

How accurate were the first pendulum clocks?

According to NIST, Huygens's first pendulum clock was accurate to within a minute a day, and later versions to within 10 seconds a day.

What is the oldest pendulum clock?

A clock by Salomon Coster dated 1657, held by the Rijksmuseum Boerhaave in Leiden, according to Guinness World Records.

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