An interesting time dilation experiment I thought of was to send a radio signal with two pulses a certain time apart to a receiver moving toward the emitter. I believe that the effect is that the accelerated reciever undergoes time dilation and would time the two pulses of light as closer together than the emitter would. I know that if I performed this experiment today, that is the result I would get but there are things I don't understand surrounding this fact. However, in this experiment I'm assuming earth is the proper rest frame. This could easily be refuted by looking at things differently. Say for instance my traveller is moving westward against the earth's spin at 100mph at the eaquator. This is equal to the speed of the earth and now my emitter is the one travelling and the reciever is still. Extend this further to the speed around the sun and say I'm travelling at 33k mph or whatever. My point is that how do I know that the sun isn't the proper rest frame? For example, in stellar aberration, the sun is the rest frame we use to calculate the expected aberration. Now that I mention stellar aberration being sun based I have to ask why it is that large sagnac devices like the ring laser project in new zealand (and michelson-gale) can detect the earths rotation on its axis which is very small in relation to the earth's movement around the sun but the device does not pick up the movement around the sun! What makes proper time proper? What makes a specific third reference point be the preferred one? How do I predict the right rest frame without arbitrarily picking the one that happens to work? I know that one explanation for my first problem is proposed to be because one of the objects has to accellerate isn't that explanation deprecated now? Unfortunately I don't know what the preferred explanation is but that older one does nothing for my other question anyway. I'm not looking for the equation for proper time; I want to understand why and how it works. Thanks in advance for your patience.