Unfortunately things with mass cannot travel at the speed of light, so we know that we are not travelling at the speed of light. Speed is all relative though, so to determine your speed you would need to measure it relative to something else, which may or may not be moving. For example, cars on the motorway are travelling at 70 mph, but only releative to stationary objects and people by the side of the road.
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Hywel commented on :
Or what Jo says of course 🙂