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For most of human history, people measured time using the sun, the stars, flowing water or burning candles. Mechanical clocks first appeared in Europe in the late thirteenth century, and they were a remarkable invention. However, they were not very accurate. Early mechanical clocks often gained or lost around fifteen minutes in a single day, and they needed to be reset regularly using a sundial. As a result, many clocks had only an hour hand, because a minute hand would have suggested a level of precision that the machine could not actually deliver. This situation changed dramatically in the middle of the seventeenth century, when the pendulum was introduced into clock design.

To understand why the pendulum mattered, it is useful to look at how earlier clocks worked. Every mechanical clock needs a source of power, such as a falling weight or a coiled spring. It also needs a part that releases this power in small, regular steps. This part is called the escapement. In the earliest clocks, the escapement was controlled by a horizontal bar called a foliot, which swung back and forth. The problem was that the foliot had no natural rhythm of its own. Its speed depended on the force pushing it, so any change in the power supply, or any increase in friction, made the clock run faster or slower.

The key to a better clock came from a scientific observation rather than from clockmaking itself. The Italian scientist Galileo Galilei noticed that a pendulum seemed to take the same amount of time to complete each swing, whether the swing was wide or narrow. According to a popular story, he first noticed this as a young man while watching a lamp moving in a cathedral in Pisa, although historians are unsure whether this account is true. In any case, Galileo studied pendulums carefully and recognised that they could be used to measure short periods of time. Near the end of his life, he even designed a pendulum clock with his son, Vincenzo. However, the clock was never finished, and Galileo died in 1642.

The first working pendulum clock was built in 1656 by the Dutch scientist Christiaan Huygens. Huygens was a mathematician and astronomer, and he needed accurate time measurement for his observations of the sky. He worked with a clockmaker in The Hague named Salomon Coster, who received the right to make the new clocks in 1657. The improvement was immediate and striking. While earlier clocks could be wrong by many minutes each day, Huygens's design was typically accurate to within about one minute a day, and later versions reduced the error to around fifteen seconds. For the first time, a clock could be trusted for days without being corrected.

The reason for this success was simple. A pendulum swings at a rate that depends mainly on its length, not on the force that drives it. In a pendulum clock, the escapement gives the pendulum a small push on each swing to keep it moving, but the pendulum itself controls the timing. Therefore, small changes in the weight or spring, or a little extra friction in the gears, no longer had a large effect on the clock's rate. In addition, owners could adjust their clocks easily. Moving the weight at the bottom of the pendulum upwards made the clock run faster, while moving it downwards made it run slower.

Huygens soon discovered, however, that Galileo's observation was not completely correct. A pendulum swinging in a wide arc takes slightly longer than one swinging in a narrow arc. This difference, now known as circular error, was small, but it was still large enough to affect the accuracy of a good clock. Huygens worked on this problem mathematically and found that a pendulum would keep perfect time if its weight moved along a special curve called a cycloid. To make this happen, he placed two curved metal plates, or 'cheeks', on either side of the point where the pendulum was hung. He published his findings in 1673 in a famous book, Horologium Oscillatorium. In practice, though, the cheeks created new problems with friction, and many clockmakers did not use them.

A more practical solution came from England around 1670 with the invention of the anchor escapement. It is often credited to the scientist Robert Hooke or the London clockmaker William Clement, but the true inventor remains uncertain. The anchor escapement allowed the pendulum to swing in a much smaller arc than before, usually only a few degrees. Because the swing was so small, circular error became almost unimportant. Furthermore, the new design worked well with long, heavy pendulums. A pendulum about one metre long takes one second for each swing, and this 'seconds pendulum' became standard. To protect these long pendulums, clockmakers placed them in tall wooden cases, which led to the popular longcase clock, later known as the grandfather clock.

The effects of these changes reached far beyond the clockmaker's workshop. As clocks became more reliable, minute hands became common, and some clocks even began to show seconds. Astronomers could record the timing of events in the sky more precisely, and ordinary households could organise their daily lives with greater confidence. Nevertheless, the pendulum clock had clear limits. It did not work well on ships, because the motion of the waves disturbed the swing, so it could not solve the important problem of finding longitude at sea. Changes in temperature also caused the pendulum rod to expand or shrink slightly, which affected its timing. These problems were only addressed in the eighteenth century. Even so, the pendulum clock remained the most accurate type of timekeeper in the world for nearly three hundred years.

How pendulum clocks improved accuracy in the seventeenth century

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