But only at zero Kelvin. Once you punch that Caesium 133 up to room temperature you need the extra 8Hz to achieve aesthetic purity. On really hot days, you need

But only at zero Kelvin. Once you punch that Caesium 133 up to room temperature you need the extra 8Hz to achieve aesthetic purity. On really hot days, you need to go to 440.1 or even 440.2.