Love this casual sentence in the New Scientist's interesting piece on the measurement of the speed of gravity. (for those of you in a rush, gravity moves very, very fast.) "He reworked the equations of general relativity to express the gravitational field of a moving body in terms of its mass, velocity and the speed of gravity. If you could measure the gravitational field of Jupiter, while knowing its mass and velocity, you could work out the speed of gravity." Yes, elementary.