It has to do with the definition of the meter. Before that, let's look at the Seconds pendulum. This is a pendulum that takes exactly 1 second to go from one side of its motion to the other (or a ...

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 ...

I said that the period of a pendulum with a length of 1 meter is 2 seconds. This would mean that pi squared would be g (the gravitational field in N/kg) - which it is. Oh, well that is just a ...

This ensures the pendulum doesn’t change length with temperature, but it still only brings the clock into the 0.1 second/day range. Clearly that’s not good enough for a clock at CERN ...