Many years ago, a patent clerk with a PhD...named Einstein, thought of the theory of Relativity.
In the Theory of Relativity it states that objects age slower the faster their velocity, and that light always travels at C...no matter your velocity, you can never catch up to a receding light.
If light can never be caught up to, then it can not be used to measure speed.
Then how, can an Earth age more than an astronaut...since neither knows which one is truly the faster velocity.
This...is Paradox #1, of Relativity. There are more to come...