How was the speed of gravity measured?
No it isn’t a coincidence, and you do not need to measure it. The fact that gravity acts at the speed of light comes straight out of Einsteins general relativity equations. It is an implication of general relativity. The equations of general relativity can be shown to be correct in the physical world, such as the long-existance of muons at high velocities, and as these equations are then correct, the implication they have that gravity acts at the speed of light must also be correct. Another way to think of it (although a bit guessing and theory) is that gravitons (theoretical, i know) would be massless, and because of this they must travel at the speed of light. However, they are a different class of particles to photons, so are not affected by barriers (ie, gravity does not change speed through media).
