Is a clock measuring the passage of time or is it defining the passage of time? Note we never observe directly "the passage of time", we only observe clocks that tick. Sometimes the clocks tick faster sometimes slower. It is an assumption that we make of physical reality that the quantity of "time" between two subsequent ticks of a clock is always the same. What we see is that motion (of observers with respect to clocks) affects the rate of ticking they see on the clocks. It is perhaps somewhat misleading or at least incomplete to say that "motion affects the passage of time" it would be more accurate to state that "motion affects the passage of time as seen by a particular observer moving with respect to the clocks being observed".

Is a clock measuring the passage of time or is it defining the passage of time? Note we never observe directly "the passage of time", we only observe clocks that tick. Sometimes the clocks tick faster sometimes slower. It is an assumption that we make of physical reality that the quantity of "time" between two subsequent ticks of a clock is always the same. What we see is that motion (of observers with respect to clocks) affects the rate of ticking they see on the clocks. It is perhaps somewhat misleading or at least incomplete to say that "motion affects the passage of time" it would be more accurate to state that "motion affects the passage of time as seen by a particular observer moving with respect to the clocks being observed". "Time itself is running slower on the moving clock" -> an observer watching a clock moving with respect to the observer will observe the clock ticking at a slower rate than a clock that is stationary with respect to the observer.

Is a clock measuring the passage of time or is it defining the passage of time? Note we never observe directly "the passage of time", we only observe clocks that tick. Sometimes the clocks tick faster sometimes slower. It is an assumption that we make of physical reality that the quantity of "time" between two subsequent ticks of a clock is always the same. What we see is that motion (of observers with respect to clocks) affects the rate of ticking they see on the clocks. It is perhaps somewhat misleading or at least incomplete to say that "motion affects the passage of time" it would be more accurate to state that "motion affects the passage of time as seen by a particular observer moving with respect to the clocks being observed". "Time itself is running slower on the moving clock" -> an observer watching a clock moving with respect to the observer will observe the clock ticking at a slower rate than a clock that is stationary with respect to the observer. "A clock that is in motion will tick off time at a slower rate" ->A clock that is in motion with respect to an observer will be observed (by the that observer) to run at a lower rate.

Is a clock measuring the passage of time or is it defining the passage of time? Note we never observe directly "the passage of time", we only observe clocks that tick. Sometimes the clocks tick faster sometimes slower. It is an assumption that we make of physical reality that the quantity of "time" between two subsequent ticks of a clock is always the same. What we see is that motion (of observers with respect to clocks) affects the rate of ticking they see on the clocks. It is perhaps somewhat misleading or at least incomplete to say that "motion affects the passage of time" it would be more accurate to state that "motion affects the passage of time as seen by a particular observer moving with respect to the clocks being observed". "Time itself is running slower on the moving clock" -> an observer watching a clock moving with respect to the observer will observe the clock ticking at a slower rate than a clock that is stationary with respect to the observer. "A clock that is in motion will tick off time at a slower rate" ->A clock that is in motion with respect to an observer will be observed (by that observer) to run at a lower rate.

We observe photons traveling at the speed of light, which, remember, is measured to be constant, taking longer to tik and to tok than the ones in the other motionless clock. Tada, relativity.