I was reading in Clifford M.Will's book "Was Einstein right? Putting General Relativity to the Test" that there was an experiment done where in October 1971 an experiment was done with radioactive clocks, and plane trips taken going with the spin of the earth, and against it. He reports: "The eastward trip took place between October 4 and 7 and included 41 hours in flight, while the westward trip took place between October 13 and 17, and included 49 hours in flight. For the westward flight the predicted gain in the flying clock was 275 nanoseconds (billionths of a second), of which two-thirds was due to gravitational blue shift; the observed gain was 273 nanoseconds. For the eastward flight, the time dilation was predicted to give a loss larger than the gain due to the gravitational blue shift, the net being a loss of 40 nanoseconds, the observed loss was 59 nanoseconds". I understood him to have explained previously that with special relativity things moving relative to you (or any other frame of reference) will undergo time dilation relative to that reference. That the reference used was an imaginary clock in the centre of the earth, and the time dilation for the clock on the earth, and the clock in the plane being calculated relative to it. When going with the spin of the earth, the clock on the plane is moving faster relative to the imagined stationary clock in the centre of the world, than it is when it flies against the spin of the earth. So the eastward trip would show more of a time slow for the plane relative to the earth (which moves the same speed relative to the centre of the earth in both trips), and in the experiment while travelling against the spin time was gained (time was going faster), when travelling with it time dilation incurred countered the gain from less gravity to give an observed loss (the time dilation was seen). What I don't understand is this can happen and time be relative. To hopefully make the problem in understanding it that I have more clear, imagine we performed 2 thought experiments, and in each we removed the planet, so we could ignore general relativity and just look at the special relativity aspect. Imagine that we just had the clock that was previously in the plane and the clock that was previously on the ground. Now relative to each other they could move as they did in the experiment in the with the spin leg. Now if things could be looked at relatively, I'd be assuming that in the first thought experiment we could imagine that the stationary point of reference was the clock that was on the ground in the experiment. We could see the two clocks together at the start, the 2nd clock leave the frame of reference and move relative to it before returning to the same frame of reference. I was thinking that since the 2nd clock would be moving relative to the first, actual time dilation for it would be expected. So it would lose time relative to the clock that was being considered to the first clock that we are imagining to be stationary. However I'm not sure why we couldn't we also have considered a different thought experiment in which we consider the 2nd clock to be the stationary frame of reference, and there the 1st clock would move relative to it, and there would be time dilation for the 1st clock, and so at the end the 1st clock would have lost time relative to the stationary 2nd clock. I guess I'm wrong because otherwise it would seem to me that the time dilation in the plane experiment showed which clock was moving relative to which (the clock in the plane underwent the time dilation relative to the clock on the Earth's surface because it was moving relatively faster), and therefore motion wouldn't be relative, and therefore nor would the time dilation. As you can probably see, I'm slightly confused on the issue, and would appreciate any help on the matter.