Is a second now defined using 9,192,631,770 Hz or 9,192,631,770.2 Hz of a caesium atom? This is obviously splitting hairs, to an extent, but I am genuinely curious... And it does make a difference...
In several places, I have seen that .2 added on to the figure...
Did the length of a second actually change a bit during its 2018 redefinition?
 A: It looks like the number 9,192,631,770.2 Hz comes from 2.75 years of measurement comparing the cesium oscillations with astronomical observation. This work was done around 1960 or so by Louis Essen of the National Physical Laboratory (NPL) in England and William Markowitz of the United States Naval Observatory. After this, the definition of the second was fixed:

At its 13th official meeting in 1967, the committee adopted the
following definition: "The second is the duration of 9,192,631,770
periods of the radiation corresponding to the transition between the
two hyperfine levels of the ground state of the caesium-133 atom."

https://www.scientificamerican.com/article/how-does-one-arrive-at-th/
So without the .2 is the definition.
A: The SI definitions are maintained by the Bureau International de Poids et Mesures (BIPM), who say

The second, symbol s, is the SI unit of time. It is defined by taking the fixed numerical value of the caesium frequency $Δν_\text{Cs}$, the unperturbed ground-state hyperfine transition frequency of the caesium-133 atom, to be $9\,192\,631\,770$ when expressed in the unit Hz, which is equal to s$^{–1}$.

Neither the 1967 definition nor the 2018 redefinition (accessible here) give the non-integer frequency in your question. That may have been a high-quality result using the previous definition of the second — which was problematic in that it required the user to go back in time and precisely measure the duration of the year 1900.
