Ethan Siegel in Forbes:
For millennia, humanity’s one-and-only reliable way to keep time was based on the Sun. Over the course of a year, the Sun, at any location on Earth, would follow a predictable pattern and path through the sky. Sundials, no more sophisticated than a vertical stick hammered into the ground, were the best timekeeping devices available to our ancestors.
All of that began to change in the 17th century. Galileo, among others, noted that a pendulum would swing with the same exact period regardless of the amplitude of the swing or the magnitude of the weight at the bottom. Only the length of the pendulum mattered. Within mere decades, pendulums with a period of exactly one second were introduced. For the first time, time could be accurately kept here on Earth, with no reliance on the Sun, the stars, or any other sign from the Universe.