• The apparently universal attractive force between matter, increasing according to the two masses multiplied together and decreasing with the square of their distance apart, according to the law of gravitation set out by Isaac Newton. The size of the force for any particular combination of distance and mass is given by the Gravitational Constant. Newton’s theory of gravitation allowed him to prove Kepler’s Laws describing the orbits of the planets and is in use today to describe orbital motion on every scale from Earth satellites to galaxies. In addition, gravitation as Newton understood it is now incorporated within the larger body of physical theory known as relativity, in which it is viewed as a form of acceleration rather than as a force, and regarded as an effect of the curvature of spacetime.