Time is usually thought of either as a separate entity that exists independently from mass and space, or as an integral part of an en-tity called space-time. Although many different kinds of clocks have been devised for measuring the passage of time, a careful look at them reveals that there are, in fact, two different arrows of time that are quite distinct from one another, and can even be said to flow at different rates. Every clock that has ever been invented measures either inertial time or gravitational time. The GPS satellites demon-strate that the rates of inertial clocks are affected both by changes in motion and changes in gravity. Thought experiments are pre-sented to show the effects of motion induced time dilation on these two different types of clocks and the actual mechanics of these changes in the rates of clocks is revealed. The conclusion is reached that there is really no physical entity called?time’ that is more than a metaphysical idea that is used to quantify different types of motion.

Keywords: gravity, clocks, time, inertia, GPS

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