When an object is at rest, it is only at rest compaired to the observer. Consider that if you are in a moving vehicle then your passenger is at rest from your point of view, but to an observer, outside the vehicle the passenger and you are both in motion. Similarly to you inside the vehicle the observer outside is moving . as the earth itself is in motion, as well as the sun and in fact the whole galaxy, their is no way to absolutely define an at rest state, so all motion is measured relative to the observer, who is assumed to be at rest compaired to the observation.
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