Galileo Galilei (1564-1642) was a great Italian mathematician who "stands at the beginning of Modern Physics".

When he was 18, Galileo's mother dragged him to Evening Service one windy night. Galileo noticed how the chandeliers were set in motion when wind escaped into the cathedral as some one entered or departed. And Galileo noticed that the swing-forward-then-backward seemed to take the SAME TIME, no matter what the LENGTH of the arc swung.

There were no efficient clocks or watches at this time. But Galileo knew he could time an activity or process by his pulse, provided he remained calm. He found approximately the same number of pulse beats for each swing of the chandelier.

Later Galileo realized that THE PENDULUM CAN BE USED TO REGULATE A CLOCK.

In his busy life, Galileo never finished developing an effective clock. But his work was completed by the great Dutch mathematician, Christiaan Huyghens (1629-1695), using a pendulum.

Still, later, other means of regulating timing-devices were introduced, including today's very accurate "atomic" clock. But this is the culmination of Galileo's discovery.

So we have clocks today because Galileo's mother dra;gged him to chruch one windy night.