<ul><li>Unix time is the number of non-leap seconds since January 1, 1970, but the rotation of the Earth is slowing down.</li><li>The second is defined in terms of the oscillations of light waves, specifically the radiation corresponding to the transition between the hyperfine levels of the ground state of the caesium-133 atom.</li><li>Cesium was chosen for atomic clocks due to its ease of isolation, and 9,192,631,770 periods of radiation were chosen for backward compatibility with the historical definition of a second.</li><li>The length of a year and the historical definition are no longer relevant, as the current definition of a second is based on the number of radiation periods.</li></ul>

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