Hi all, I have a problem to fully understand how we can apply Special Relativity to a system where one observer is still in the center, and other one is moving in a circle around. For example, like a satellite orbiting Earth. In case of GPS, the clocks carried by satellite are running slower than clocks located on the ground - as a result of time dilation predicted by Special relativity due to their relative motion. (I know there is opposite and even more significant influence due to General relativity, but let's ignore this one). Now to my question... My understanding is that SR is correct only for observes within inertial frames of reference, i.e. non-accelerating frames of reference. However, in case of satellite orbiting Earth, isn't there a permanent centripetal acceleration produced by gravitational force of the Earth (causing the curved/orbital path of the satellite)? Then how can we use SR to predict time dilation as described above? I have two ideas in my mind, how can that be, but not sure whether any of them is correct to answer my question: 1) The orbital path is so large, so at any given infinitesimal time interval it can be considered as straight path. And consequently the relative motion can be seen as straight uniform motion. 2) Even there is the centripetal force acting on the satellite, it's frame of reference can be considered as inertial - it is in a free fall, and there won't be any net force acting on it. Thanks in advance for your help!