The Michelson-Morley Experiment showed that light velocity had the same magnitude in different directions.
One metre is defined to be the length of the path travelled by light in a vacuum in 1/299 792 458 seconds. Therefore,
c = 299 792 458 metres per second. The second is defined in terms of the frequency of electromagnetic radiation emitted during a transition between two specific states in a caesium atom. In other words, it's a unit of time as measured on a kind of atomic clock, which by convention is constant.
It's an open question whether a second on a pendulum clock is the same as one measured on an atomic clock for all time, as that depends on G.
One metre is defined to be the length of the path travelled by light in a vacuum in 1/299 792 458 seconds. Therefore, c = 299 792 458 metres per second. The second is defined in terms of the frequency of electromagnetic radiation emitted during a transition between two specific states in a caesium atom. In other words, it's a unit of time as measured on a kind of atomic clock, which by convention is constant.
It's an open question whether a second on a pendulum clock is the same as one measured on an atomic clock for all time, as that depends on G.