A reliable swing
Why Does a Pendulum Keep Time?
A pendulum's swing takes the same time whether it swings wide or narrow.
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Step 1 of 5
A pendulum is a weight on a string.
Pull it aside and let go. Gravity pulls it back toward the middle, it overshoots, and it swings out the other side. Then it repeats.
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Step 2 of 5
Energy swaps back and forth.
At the top of a swing it is high and still — all stored energy. At the bottom it is low and fast — all movement energy. Then it trades back.
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Step 3 of 5
Bigger swings are not slower swings.
Pull it further and it travels a longer arc — but it also moves faster. The two effects cancel, so the time per swing stays the same.
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Step 4 of 5
Only the length matters.
The swing time depends on the string's length and nothing else. A heavier bob does not change it. A one-metre pendulum takes almost exactly one second each way.
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Step 5 of 5
That reliability made accurate clocks possible.
Before pendulums, the best clocks lost 15 minutes a day. After Huygens used one in 1656, clocks lost about 15 seconds a day.
Galileo spotted this as a teenager, reportedly by timing a swinging cathedral lamp against his own pulse.
The short version
A pendulum keeps time because its swing period depends only on its length, not how far it swings.
Try it yourself
Tie a key to a 25 cm thread and swing it. Now try a much wider swing. Count — the rhythm does not change.
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