The one that flies. A term applicable to the heavens loosely. Milton thinks about the atmospheric air; he's the pope of ambient clouds. It was used by Ptolemy to identify the tenth sphere that moved all other spheres with it from the East into the heavens through its persuasive power.
The contemporary astrologer is free to apply this excellent definition to the surrounding magnetic field of the Earth as variously charged by the movements of the planets, the Sunspot period, and other astronomical phenomena after Copernicus exploded this idea.