I was talking to someone online the other day, and they were claiming that general relativity must be wrong because it predicts that gravity must propagate at the speed of light, and he said this must be wrong, because if that were the case the earth would be attracted to the place where the sun was eight minutes ago, and so orbit would not be possible. I thought to myself 'hahaha you silly rabbit', I can easily answer that, and so I explained to him that it would be just the same situation as when if you tie a weight to the end of a string, and swing it round your head at a constant angular velocity, and if you start walking at a constant speed, the orbit of the weight could still be maintained at the same angular velocity, and that the two situations would be indistinguishable, and as the force cannot be propagted through the string instantaneously, this answers his query. He couldn't really answer this point, but then later I started thinking more about this, and I realised my analogy was incorrect. The force vector in the string always points down the string towards the centre of motion, and the point is the string itself is also moving at a constant angular velocity, thereby ensuring that the force and centripetal acceleration vector is always pointing in to the centre of the motion. This is not the case for the orbit of the earth however, as there is no 'string' that is moving round with us. Firstly of course, we need to determine a frame of reference. If you are on the earth or on the sun (ouch, hot!) there is no problem as the solar system is basically at rest from your perspective. However lets imagine you are sitting on some rocky body, lets say 50 light minutes away looking down at the plane of the solar system as it goes moving by you at a constant velocity. Surely if gravity propagates at the speed of light, then the force vector on the earth will be pointing to where the sun was eight minutes ago? I'm sure my crazy friend is wrong, but I am stumped as to how, can anyone help?