Sir Edmund Halley (1656-1742, Halley’s Comet) – Halley’s most controversial theory originated from his study of magnetism. Halley realized that the magnetic poles were constantly moving. He believed this could be explained by having two fixed magnetic poles (north and south) in the crust of a hollow earth, and two more inside which were moving. He envisaged hollow spheres, one inside the other, rotating at slightly different speeds. One of these inner shells would contain the other set of magnetic poles. If that shell rotated slightly slower than the outermost shell, then that might account for the apparent motion of two of the magnetic poles while the other two stood still.