system
Given the following post, respond as if you're commenting on Reddit:
human
How accurate can we get when measuring the speed of light? Wouldn't time be slower in one zone than another zone as the light travels, therefore make the measurements of it inconsistent?

A metre is defined by the light, so the speed of light has no decimal points. So I was wondering if measurement of time is also define by the light as well. If not, would that not make it inconsistent, therefore cannot definitively measure it?
gpt
Time is defined by atomic clocks. Specifically it's "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". Basically, an electron in this particular element will bounce back and forth between two nearby energy levels at a known frequency of 9,192,631,770 Hz, so we use 9,192,631,770 wibbles of this atom to define the second.

Once you have the second defined, you can measure how far light travels in 1/c seconds (in a vacuum under ideal circumstances), and that's the definition of a metre.