Caesium atomic clock readings. A caesium atomic clock contains atoms of vapourised caesium-133 that pass back and forth between magnets at each end of the clock. The caesium-133 atoms oscillate between two energy levels as they go. Counting these oscillations is the basis of the standard second, where one second is about 9193 million oscillations. The chart shows that, according to readings from this clock, the day length has varied over ten years but gives an overall increase of about two milliseconds during this time.