A word for the firmament or “the heavens” (but not “Heaven”), derived from the root √MENEL of the same meaning (PE17/24, 152; PE21/84), possibly √MEN + √EL “✱direction of the stars” (RGEO/64). It was often used in contrasted to cemen, “earth”, “the earth” (but not “Earth”), as the surface of the world versus its (apparent) “roof”. Tolkien clarified that “these were ‘pictorial’ words, as the lore of the Eldar and the Numenoreans knew much astronomy” (PE17/24), so that menel as the dome over the world was metaphorical rather than actual.
At one point in the legendarium there was an actual dome of heaven created by Varda, but over Aman rather than the entire world:
> Later, when the Valar took refuge from Melkor, and the imminent ruin of Arda, and built and fortified Valinor in Aman, it was Varda who made the great dome above Valinor, to keep out any spirits or spies of Melkor. It was made as a simulacrum of the true firmament (Tar-menel), and the patterns were therein repeated, but with apparent stars (or “sparks”: tinwi) of greater relative size to the total visible area. So that the lesser firmament of Valinor (Nur-menel) was very brilliant (PE17/22; MR/388).
This notion was not mentioned in The Silmarillion as published, however.
Note that Quenya does not use the definite article before unique individual things like the Sun and the Moon. Likewise menel “the heavens” does not require the definite article, and i menel is almost always wrong.
Conceptual Development: At various points in his work on the legendarium Tolkien considered having a literal firmament or “upper airs”, ᴱQ. Vaitya (QL/100) >> ᴹQ. Vaiya. See for examples, his diagrams of the world in the Ambarkanta (SM/243, 245). How literal this notion was depended on whether Tolkien was considering the legendarium as a cycle of myth or as an actual description of the world. In earlier documents the term for the firmament or “outermost airs” was often ᴱQ. Vaitya >> ᴹQ. Vaiya (QL/100; SM/241-245) or Q. Ilmen (SM/241-245, LR/12 etc.), the last term surviving into The Silmarillion as published (S/99).
The term menel appeared in the 1940s, mentioned as “heaven” in an early draft of Lord of the Rings Book I (RC/671) and also appearing in The Notion Club Papers and related document of the 1940s where it was a cognate of Ad. minil or minal and derived from the ancient Elvish root ᴹ√MENEL (SD/241, 414). In these document it was distinctly “the heavens, the firmament” (SD/401), but in the 1950s Quenya prayer Átaremma, Tolkien used menel = “Heaven” a number of times (VT43/8-12), though in the final draft of the prayer he used the term Eruman for “Heaven” (VT43/12). For the most part, though, Tolkien used menel for “the heavens” rather than “Heaven” in later writings.
Menel noun "heavens" (Markirya, SA), "the heavens, the firmament" (SD:401), "the apparent dome in the sky" (MR:387). Menel Cemenyë ("k") "Heaven and Earth" (VT47:30). Found in names like Meneldil "Heaven-friend" = astronomer (Appendix A; Letters:386), Meneldur masc. name, "Heaven-servant" (Appendix A, Tar-Menelduras a Númenórean King, UT:210); menelluin "sky-blue", used as noun = "cornflower" (J.R.R. Tolkien: Artist & Illustrator p. 193). Menelmacar "Swordsman of the Sky", the Orion constellation (also called Telumehtar, Appendix E, first footnote); the older name was Menelmacil "Heaven-sword" (WJ:411); Meneltarma "Pillar of Heaven", name of the great central mountain of Númenor (SA:tar, VT42:21).Menelya fifth day of the Eldarin six-day week, dedicated to the heavens (Appendix D) Locative meneldë "in heaven"; abandoned forms menellë, menelzë (VT43:12, 16). Adj. meneldëa "(being) in heaven", evidently based on a locative form meneldë "in heaven"; abandoned forms menelzëa, menellëa, menelessëa (VT43:13, VT44:16; the last of these forms would suggest the locative form #menelessë).