isochronism

Isochronism says that the time it takes an oscillator to complete one full cycle does not depend on how big the swing is. Small swings and large swings take the same amount of time.

For a pendulum this is only exactly true in the small-angle limit, where sin θ can be replaced by θ. Push the amplitude past about fifteen degrees and the period starts to stretch measurably. But in the small-angle regime, the effect is invisible to the naked eye — which is exactly why Galileo noticed it from a chandelier in a cathedral.

Galileo observed isochronism in 1583 while watching a bronze chandelier sway in Pisa cathedral. He timed its swings against his pulse and found them constant regardless of amplitude. The discovery led, within a generation, to the pendulum clock.