And there is on this island a magnificant precinct sacred to Apollo and a notable spherical temple decorated with many votive offerings. There is also a community sacred to this god where many of the inhabitants ... worship the god with songs celebrating his deeds ... It is said that the god returns to the island every 19 years, the period in which the return of the stars to the same place in the heavens is accomplished. At the time of the appearance, the god plays on the lyre and dances continually by night from the spring equinox until the rising of the Pleiades.
Diodorus Siculus Histories Book V (50-30BC)
paraphrasing Hecateus (330BC) in what
may be the earliest documented description
of Stonehenge
Rodney Castleden comments on the word "spherical" (p240) and suggests the word implied is 'astronomical' referring to the celestial sight lines contained within the construction of the stones rather than referring to its circular shape as some have thought.
Taking an excerpt from a book with its feet firmly in the fringe (The Sphinx and the Megaliths by John Ivimy (Abacus) (1976) p91), I found that this author takes a different slant. The central postulate behind this book is that Stonehenge was built by an Egyptian colony. By quoting him I am not suggesting that I agree with any part of what he says, I am just interested in what people have had to say with the fringe idea of a roof on Stonehenge over the years.
First John Ivimy gives us a little more from Diodorus:
"Of those who have written about the ancient myths (relating to the Hyperboreans) Hecateus and certain others say that in the regions beyond the land of the Celts there lies in the ocean an island no smaller than Sicily. This island, the account contnues, is situated in the north because their home is beyond the point whence the north wind (Boreas) blows;" You can find more quotes from Diodorus on the web (e.g. "The Eternal Idol" website (including the original Greek if you prefer), where it suggests the word spherical can also mean "vaulted").