Kepler believed that the sun did not sit passively at the center of the solar system but that through some mysterious power or "virtue" actually compelled the planets to hold to their orbits. Because the planets moved slower when they were farther from the sun, this power must diminish with increasing distance. The idea that the planets were controlled by the sun was developed by Isaac Newton in his laws of motion and law of gravitation. Newton assumed that the sun continuously exerts a force on each planet that pulls the planet toward the sun. He calculated that elliptical orbits would result if the force varied inversely as the square of the distance from the sun (i.e., when the distance doubles, the force becomes four times weaker). His law of universal gravitation predicts that the planets exert small forces on each other although subject to the dominant force of the sun. These small additional forces explain most of the small departures from Kepler's laws revealed by later, more accurate observations.