Quite right Adrian, the earth's rotation is never constant, in fact, it's rotation speeds up and slows down and any clock, atomic or otherwise, will be out of sync with another by as much as 0.09836363636... seconds at any particular time. Scientists have found that when certain cities "spin" past (in line with) the sun, the rotation speeds up and vice versa when these cities are opposite the sun the rotation speed slows down. It's a phenomenon know as "Axis Displacement", when building materials are taken from, say, China and used to build a massive structure in, say, USA, the balance of earth's surface has been changed, this causes a "wobble" on the rotational axis and this causes the speeding-up and slowing-down in rotation. Axis Displacement can effect the time by about 0.098363636363... seconds. It isn't a proven theory though, (yet).
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