Chemistry / Unit 1 Chem 0093 short lesson, about 20 seconds
The Second
Captions available
The idea
The second, SI's base unit of time, is defined by counting a fixed number of cesium atom radiation oscillations.
In practice
An atomic clock keeps time by counting these oscillations directly, not by any mechanical or astronomical reference.
Where people slip
The second is not any fraction of a day, though that is where its rough size historically came from. (NIST/BIPM, Aug 20, 2026.)
Your place on the trail
Every lesson rests on earlier ones and opens later ones. Tap any to jump.
Builds on
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