Roman writer Marcus Terentius Varro (116-27 BC) is an important source of information about language and religion during the first century BC. He was the author of about 620 books, including 25 books on Latin, and many more on religion, mythology, antiquities, and other subjects. Centuries later, Augustine referred to him as "a highly intelligent and learned writer," and a man with a "first-rate mind."
In his treatise On the Latin Language [V, 19], Varro gave his views about the origin of the word caelum:
On the whole I think that from chaos came choum and then cavum 'hollow,' and from this caelum 'sky,' since, as I have said, 'this around and above, which holds in its embrace the earth,' is the cavum caelum 'hollow sky.' and so Andromeda says to Night, 'you who traverse the hollows of sky, with your chariot marked by the stars.' And Agamemnon says, 'In the shield of the sky, that soundeth on high,' for a shield is a hollow thing. And Ennius likewise, with reference to a cavern, 'Enormous arches of the sky.'
The Romans, he said, identified the sky with Jupiter and Earth with Juno: "These same gods Sky and Earth are Jupiter and Juno..." and he quoted Ennius who said Jupiter is called air by the Greeks, and is identified with wind and cloud, rain, and cold. Varro added: "Because all come from him and are under him, he addresses him with the words: 'O father and king of the gods and the mortals.' [On the Latin Language, V, 65.]
Varro explained that the name 'Jupiter' was derived from "Diespaiter, that is, 'Father Day;' from which they who come from him are called dei 'deities,' and dius 'god' and divum 'sky,' whence sub divo 'under the sky' and Dius Fidus 'god of faith.' Thus from this reason the roof of his temple is pierced with holes, that in this way the divum, which is the caelum 'sky,' may be seen. Some say it is improper to take an oath by his name, when you are under a roof." [On the Latin Language V, 66.]
Varro continued, "Because Juno is Jupiter's wife, and he is Sky, she Terra 'Earth,' the same as Tellus 'Earth,' she also, because she iuvat 'helps' una 'along' with Jupiter, is called Juno, and Regina 'Queen,' because all earthly things are hers." [On the Latin Language V, 67.] He showed Apollo was the Greek name for the sun, and that the Romans called the moon Diana. According to Varro, the sky was also called templum, the temple of Iovis or Zeus. He cited the poets: "One there shall be, whom thou shalt raise up to sky's azure temples.. " and, "Trembled the mighty temple of Jove who thunders in heaven..." [On the Latin Language VII, 6-7.]
Varro also helps to clear up the question of whether Olympus meant the sky, or the name of a mountain. He wrote [On the Latin Language VII, 20]:
Olympus is the name which the Greeks give to the sky, and all peoples give to the mountain in Macedonia; it is from the latter, I inclined to think, that the Muses are spoken of as Olympiads...
Varro does not associate the word firmamentum with the sky.