The Earth is constantly changing direction in it’s orbit, as well as spinning on it’s axis – so as Brian stated, any observer on the surface is not in constant velocity but is in fact always accelerating – therefore the observer is technically never in an inertial frame of reference.

Thanks Brian – engaging intro to this topic which has always fascinated me. My ongoing question about Special Relativity is whether consideration of the constant speed of light changes our perception of reality (that is, distance, time and mass relative to us) or whether it actually reveals our misunderstanding of ‘true’ reality.