Speaking of æther...*
In Greek mythology, Æther was the offspring of Erebus (deep shadow) and Nyx (night). Despite springing from dark parents, the word is related to the verb that means "to incinerate";** "æther" was used to refer to pure fresh air, something more pure than ordinary air; in fact, a pure air that was breathed by the gods.
Plato's Timæus (which was very popular for medieval scholars, as I've mentioned before) and his student Aristotle both considered æther crucial to the structure of the universe. Aristotle called it the "fifth element" and described its superiority over earth, air, fire and water because it did not have their limiting properties (hot or cold, wet or dry) and was unchangeable. It was also called quintessence which means "fifth essence/element."
Small wonder then that the Middle Ages filled the area above the earthly atmosphere, the space through which the celestial spheres rotated and planets and stars moved, with æther. Æther could not be disproved, and the vacuum of space was as difficult to imagine for the Middle Ages and later as "zero" was for the Romans earlier. The 17th century philosopher Robert Fludd fused Plotinus and Genesis when he explained:
The middle region of the universe, created on the second day, has various names because of the action of the light-stuff as it extended downwards; for, taken by itself, with regard to its own particular material, it is called the Middle Spirit, after the dispersal of darkness: compared to the upper sky, that is, to light-stuff, or mixture of light-stuff and spirit, it is called Ether... [Robert Fludd, The Technical, Physical and Metaphysical History of the Macrocosm and Microcosm, 1617-1624]The scientific theories were there for all to read and understand.
To be totally honest, the "ether" being tested for in the Michelson-Morley experiment was not conceptually the same æther discussed so extensively in classical and medieval times, but the descent from one to the other clearly runs through the philosophical writings of Western Civilization. Æther was here to stay, until modern technology could eliminate it from our worldview.
*With a nod to Brian Koberlein (on Google +) for shamelessly stealing this idea and his title.
**The name Æthiopia was coined because the inhabitants were black-skinned, as if burnt by the sun.