I imagine the
Silmarillion as something like Ovid’s
Metamorphoses. Ovid begins his work with a story of creation. But the story he tells is not based on any mythological account, but on more advanced philosophical speculations, principally on the speculations of Hērákleitos. This name was Latinized to
Heraclitus and his teachings were particularly honored by the Stoics. Stoicism was the foremost popular philosophy among the educated elite in the Hellenistic world and the Roman Empire. After the 5th century ʙᴄᴇ, no Greek writer of repute thought the world was anything but round.
So Ovid here, following the science of his time, naturally imagines that his Earth is spherical in shape. See
http://www.sacred-texts.com/cla/ovid/meta/meta01.htm.
Yet when telling the story of Phaéthōn, the son of the Sun god (see
http://www.sacred-texts.com/cla/ovid/meta/meta02.htm), whose name Ovid Latiinizes as
Phaeton, Earth is conceived of as flat and the Sun god is conceived of as dwelling, before sunrise, in a marvelous palace at the east of the flat earth.
Apparently, like Ovid’s
Metamorphoses, or like Apollódōrus’
Bibliothḗkē (‘The Library’), the
Silmarillion was conceived by Tolkien as a work of great prestige in Gondor, prestige that meant it continued to be copied although it was known that ‘the Wise of Númenor’ found many errors in it. It is not surprising that a least one copy, or many copies, would be found in the library at Rivendell for Bilbo to translate.
Tolkien writes, as published on page 374 of
Morgoth’s
Ring (HoME 10):
The cosmogonic myths are Númenórean, blending Elven-lore with human myth and imagination. A note should say that the Wise of Númenor recorded that the making of stars was not so, nor of Sun and Moon. For Sun and stars were all older than Arda. But the placing of Arda amidst stars and under the [?guard] of the Sun was due to Manwë and Varda before the assault of Melkor.
Tolkien does not indicate whether this note, and presumably other notes, was to be represented as a translation from the source written in Sindarin, as a note by Bilbo, or as a note by himself.