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<-   The units used to measure time   ->

The time unit of the International System is the second.

The main units of time in astronomy are the second (defined by the vibrations of the Cesium atom) and the year (defined by the Sun). Time is given in Universal Time (UT), which is the mean time on Earth, at zero longitude.

figures/unjourunan.png
The Earth's rotation around its axis defines the day and the revolution of the Earth around the Sun defines the year.
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