hellë noun "sky" (3EL; a distinct word hellë "frost" was struck out, see KHEL.)
Noldorin
ell
noun. sky
Primitive elvish
menel
noun. heaven, sky
Sindarin
Menel
noun. sky, high heaven, firmament, the region of the stars
Quenya
hellë
sky
ilwë
sky, heavens
ilwë noun "sky, heavens" (LT1:255), "the middle air among the stars" (LT1:273). VT49:51, 53 also mentions an obscure prononominal element ilwë.
Beware, older languages below! The languages below were invented during Tolkien's earlier period and should be used with caution. Remember to never, ever mix words from different languages!
Gnomish
climli
noun. heaven, sky
A noun in the Gnomish Lexicon of the 1910s glossed “heaven, sky”, derived from the root ᴱ√tel- (GL/26). It is likely related to ᴱQ. telimbo “canopy, often used = sky” (QL/90), where the initial tel- became tl- and then tl- became cl-.
dai
noun. sky
ilon
noun. sky
A word for “sky” in an early name list from the 1910s and appearing in a couple early names: G. Thlim Quing Ilon “✱Folk of the Heavenly Arc” and G. Cris o Teld Quing Ilon “Gully of the Rainbow Roof” (PE13/101, 104), the latter revised to G. Cris Ilbranteloth (LT2/202). In the contemporaneous Gnomish Lexicon, however, G. Ilon was the name of Ilúvatar (GL/50).
luithon
noun. sky
A word for “sky” in the Gnomish Lexicon of the 1910s, an elaboration of G. lui “blueness” (GL/55).
Early Noldorin
telum
noun. sky, sky; [G.] roof
Early Quenya
taime
noun. sky
A word for “the sky” in the Qenya Lexicon of the 1910s, with variants Taime and Taimie from the early root ᴱ√TAHA [DAHA] (QL/87). ᴱQ. tea “sky” from Early Qenya Word-lists of the 1920s is probably related, as suggested by Patrick Wynne and Christopher Gilson (PE16/142).
taimie
noun. sky
tea
noun. sky
Qenya
helle
noun. sky
A noun in The Etymologies of the 1930s glossed “sky” derived from the root ᴹ√ƷEL of the same meaning (Ety/ƷEL).
Doriathrin
gell
noun. sky
A Doriathrin noun for “sky” derived from the root ᴹ√ƷEL (Ety/ƷEL), probably from a primitive form ✱✶ʒellē [ɣellē] based on its cognates. It is a clear example of how [[ilk|initial [ɣ] became [g]]] in Ilkorin.
Old Noldorin
el
noun. sky
elle
noun. sky
Middle Primitive Elvish
menel
root. heaven, sky
ʒel
root. sky
A root in The Etymologies of the 1930s glossed “sky”, with derivatives like ᴹQ. helle/N. ell “sky” and ᴹQ. helwa/N. elw “(pale) blue” (Ety/ƷEL). It was the basis for the initial elements of the names N. Elrond, N. Elwing and ᴹQ. Elwe, but elsewhere Tolkien connected these names to √EL “star”. It was also an element in the word N. eilian(w) “rainbow, (lit.) sky-bridge”, later given as S. ninniach. On the basis of these changes, I think it is likely Tolkien abandoned ᴹ√ƷEL, but some of its derived words are still popular in Neo-Eldarin.
Primitive adûnaic
minil
root. heaven, sky
A Primitive Adûnaic root glossed “heaven, sky” that Tolkien used as an example of an “early borrowing” from Primitive Elvish: the Primitive Elvish root √MENEL became ✶Ad. √MINIL from whatever Avari language influenced Primitive Adûnaic (SD/414).
An element meaning “sky” in several names from The Etymologies of the 1930s: N. Elfaron “Sky-hunter” (Ety/SPAR) and N. Elthoron “Eagle of the Sky” (Ety/THOR). It was derived from the root ᴹ√ƷEL “sky” which had an Old Noldorin form: ON. elle (Ety/ƷEL). However, Tolkien said “In Noldorin and Telerin this is confused with EL star”, implying that the word was not used in modern language; an earlier but rejected version of this entry had archaic N. †ell, el “sky” (EtyAC/ƷEL).
Neo-Sindarin: Despite the above statements, ell is probably the best attested option for “sky” in Neo-Sindarin, and I would use it as such, since it is in fact distinct from S. êl “star”, a word that is itself archaic/poetic versus more common S. gil.