[carljoseph]

The definition of a second isn't based on the Earth's motion, but some natural phenomena like you recommended. "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." [1]

[1] https://www.scientificamerican.com/article/how-does-one-arri...

[saagarjha]

Of course, that definition was made to make our current definition of the second continue to work.