Hello, I noticed quite a few questions on the Hafele-Keating experiment here, but I believe none that have my exact question. I understand the explanation given for "a frame of reference at rest with respect to the center of the earth", as in the Wikipedia article. But what happens if we consider, say, the eastbound plane as the reference frame? Then the clock in there becomes the reference clock. The clock on earth moves away from the plane, in the plane's reference frame. So does the clock on the westbound plane -- even faster. Should we conclude that the clock on earth will tick more slowly than the reference clock, and the clock on the westbound plane more slowly still? But that is not what the experiment showed. Furthermore, if you do the same with the westbound plane as the reference frame, then it goes the other way around: now the earth clock and eastbound clock would be expected to be slower. (I don't see that either the west or east - or any other - direction is 'preferred' in some sense.) Maybe the answer is that, counter-intuitively, everybody is right - those who conclude from the westbound plane, earth, and eastbound plane reference frames are all correct, even though they give conflicting results. But then: surely (??!) the clocks are devices that can only show a single value -- and in the experiment they showed those that correspond to the earth reference frame. So... what am I missing? Thanks!