Since the attraction between nucleus and electron, like that between two planets, varies inversely as the square of the distance, the electrons can move not only in circular but also in elliptical orbits. But the elliptical orbits are likewise governed by the rules of the quantum theory. The major axes of all possible ellipses belonging to one shell of electrons are all equal, in fact they are equal to the diameter of the circular orbit for that shell. The minor axis is always a simple fractional part of the major axis. Each different orbit corresponds to a different energy content potential and kinetic, so that the passage from one orbit or another means that energy is absorbed or emitted. When an electron moves to a more remote orbit, or, as we should say with respect to a terrestrial body it is raised, energy must be supplied to the atom.