Michael 345; Note, a 1 second cycle is measured with an extremely smaller cycle. (wave length)=c/frequency The total length of 9 billion+ wave lengths is 1 light second, 186,000 miles or 3(10)^8 meters. I agree with you that measurements use arbitrary units. We could shoot an arrow a distance d into a distant tree, place a mirror on the tree, and reflect a light pulse over a distance 2d. Then define a new um for light transit time t as d/c. The tree would be 1 arrow distant. It's always what is simple and convenient.