The semiclassical hydrogen atom is called the Sommerfeld model, and its orbits are ellipses of various sizes at discrete inclinations. The Sommerfeld model predicted that the magnetic moment of an atom measured along an axis will only take on discrete values, a result which seems to contradict rotational invariance but which was confirmed by the Stern–Gerlach experiment. This Bohr–Sommerfeld theory is a significant step in the development of quantum mechanics. It also describes the possibility of atomic energy levels being split by a magnetic field (called the Zeeman effect).