Question: Who believed the celestial region was made up of Aether?
About the article "Astrophysics", section "History".

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In Plato's "Timaeus" (55d) speaking about air, Plato mentions that "there is the most translucent kind which is called by the name of aether (αίθηρ)".
Aristotle, who had been Plato's student at the Akademia, disagreed with his former mentor and added "aether" to the system of the classical elements of Ionian philosophy as the "fifth element", He noted that the four terrestrial classical elements were subject to change and naturally moved linearly.
Aether however, located in the celestial regions and heavenly bodies, moved circularly.
In Aristotle's system of classical elements, aether had none of the qualities the terrestrial classical elements had.
Aether was neither hot nor cold, neither wet nor dry.
Aether did not follow Aristotelian physics either.
Aether was also incapable of motion of quality or motion of quantity.
Aether was only capable of local motion.
Aether naturally moved in circles, and had no contrary, or unnatural, motion.
Aristotle also noted that crystalline spheres made of aether held the celestial bodies.
The idea of crystalline spheres and natural circular motion of aether led to Aristotle's explanation of the observed orbits of stars and planets in perfectly circular motion in crystalline aether.
Medieval scholastic philosophers granted "aether" changes of density, in which the bodies of the planets were considered to be more dense than the medium which filled the rest of the universe.
Robert Fludd stated that the aether was of the character that it was "subtler than light".
Fludd cites the 3rd-century view of Plotinus, concerning the aether as penetrative and non-material.
See also Arche. 
According to Aristotle, the elements which compose the terrestrial spheres are different from that constituting the celestial spheres.
He believed that four elements make up everything under the Moon, i.e.
everything terrestrial: earth, air, fire and water.
He also held that the heavens are made of a special weightless and incorruptible (i.e.
unchangeable) fifth element called "aether".
Aether also has the name "quintessence", meaning, literally, "fifth substance".
Aristotle considered heavy substances such as iron and other metals to consist primarily of the element earth, with a smaller amount of the other three terrestrial elements.
Other, lighter objects, he believed, have less earth, relative to the other three elements in their composition. 
  aether (classical element), the material supposed to fill the region of the universe above the terrestrial sphere   Aether (mythology) was the personification of the "upper sky", space and heaven, in Greek mythology  
In considering the physics of the celestial spheres, scholars followed two different views about the material composition of the celestial spheres.
For Plato, the celestial regions were made "mostly out of fire" on account of fire's mobility.
Later Platonists, such as Plotinus, maintained that although fire moves naturally upward in a straight line toward its natural place at the periphery of the universe, when it arrived there, it would either rest or move naturally in a circle.
This account was compatible with Aristotle's meteorology of a fiery region in the upper air, dragged along underneath the circular motion of the lunar sphere.
For Aristotle, however, the spheres themselves were made entirely of a special fifth element, "Aether" (Αἰθήρ), the bright, untainted upper atmosphere in which the gods dwell, as distinct from the dense lower atmosphere, "Aer" (Ἀήρ).
While the four terrestrial elements (earth, water, air and fire) gave rise to the generation and corruption of natural substances by their mutual transformations, aether was unchanging, moving always with a uniform circular motion that was uniquely suited to the celestial spheres, which were eternal.
Earth and water had a natural heaviness ("gravitas"), which they expressed by moving downward toward the center of the universe.
Fire and air had a natural lightness ("levitas"), such that they moved upward, away from the center.
Aether, being neither heavy nor light, moved naturally around the center. 
Before 1887 it was generally believed that light travelled at a constant speed relative to the hypothesised medium of the aether.
For an observer in motion with respect to the aether, this would result in slightly different two-way speeds of light in different directions.
In 1887, the Michelson–Morley experiment showed that the two-way speed of light was constant regardless of direction or motion through the aether.