The swinging lamp in Pisa Cathedral

The story goes that in 1583, nineteen-year-old Galileo was sitting in Pisa Cathedral when he noticed a lamp swinging from the ceiling. Sometimes the swings were wide, sometimes small.

He had no clock, so he timed the swings against his own pulse. To his surprise, each swing seemed to take the same time, no matter how far it travelled. A wide arc was simply faster than a short one.

The tale comes from his first biographer, Vincenzo Viviani, and may have been polished. But Galileo did study pendulums seriously, finding that the time of a swing depends on the length of the string, not the weight at the end. Near the end of his life, blind and under house arrest, he designed a pendulum clock with his son.

In 1656 Christiaan Huygens built the first working pendulum clock, cutting errors from around fifteen minutes a day to about fifteen seconds. Galileo’s real discovery was a habit: when everyone else sees something ordinary, measure it.