Chemistry / Chem 0093 · Atom · ~20 seconds
The Second
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The second, SI's base unit of time, is defined by counting a fixed number of cesium atom radiation oscillations.
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An atomic clock keeps time by counting these oscillations directly, not by any mechanical or astronomical reference.
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The second is not any fraction of a day, though that is where its rough size historically came from. (NIST/BIPM, Aug 20, 2026.)