Second (s)

Quantity: Time · Category: SI base

The second is the SI base unit of time, defined by the cesium atom: exactly 9,192,631,770 oscillations of the radiation from a cesium-133 atomic transition. This atomic definition keeps time uniform to better than a second in tens of millions of years.

Relation to SI

Base unit — defined via Δν_Cs = 9,192,631,770 Hz exactly

Examples

• A resting human heartbeat takes roughly 0.8 seconds.
• GPS satellites must account for billionths of a second to locate you accurately.