In The Etymologies (Ety/358, corrected by VT/45:15), this word was given as geil , plural gîl. However, later in LotR/E and RGEO/73, Tolkien seems to consider gil as a singular (with no hints in the sources of what the plural would be, besides the collective plural giliath )
Sindarin
gil
noun. star; (bright) spark, silver glint, twinkle of light
gil
noun. star, bright spark
gil-
noun. star
giliath
noun. all the host of stars
gill
noun. star
gîl
silver glint
(i ngîl = i ñîl, o n’gîl = o ñgîl, construct gil) (star, bright spark), no distinct pl. form except with article (in gîl = i ñgîl), coll. pl. *giliath (RGEO, MR:388)*
gîl
star
gîl (i ngîl = i ñîl, o n**gîl, construct gil) (bright spark, silver glint), no distinct pl. form except with article (in gîl = i ñgîl), coll. pl. giliath** (RGEO, MR:388). Poetic †êl (elen-, pl. elin, coll. pl. elenath) (RGEO, Letters:281, WJ:363).
gîl
star
(i ngîl = i ñîl, o n’gîl, construct gil) (bright spark, silver glint), no distinct pl. form except with article (in gîl = i ñgîl), coll. pl. giliath **(RGEO, MR:388). Poetic †êl (elen-, pl. **elin, coll. pl. elenath) (RGEO, Letters:281, WJ:363).
gîl
bright spark
(i ngîl = i ñîl, construct gil) (star, silver glint), no distinct pl. form except with article (in gîl = i ñgîl), coll. pl. *giliath** (RGEO, MR:388)*
gîl
silver glint
gîl (i ngîl = i ñîl, o n**gîl = o ñgîl, construct gil) (star, bright spark), no distinct pl. form except with article (in gîl = i ñgîl), coll. pl. giliath** (RGEO, MR:388)
gîl
silver glint
gîl (i ngîl = i ñîl, o n**gîl = o ñgîl, construct gil) (star, bright spark), no distinct pl. form except with article (in gîl = i ñgîl), coll. pl. giliath** (RGEO, MR:388).
gîl
bright spark
gîl (i ngîl = i ñîl, construct gil) (star, silver glint), no distinct pl. form except with article (in gîl = i ñgîl), coll. pl. giliath (RGEO, MR:388)
gîl
bright spark
gîl (i ngîl = i ñîl, o n**gîl = o ñgîl, construct gil) (star, silver glint), no distinct pl. form except with article (in gîl = i ñgîl), coll. pl. giliath** (RGEO, MR:388)
gail
bright
gail (light), lenited ngail; no distinct pl. form (VT45:18). The adj. calen etymologically means "bright", but is used = "green" (q.v.).
gail
bright
(light), lenited ngail; no distinct pl. form (VT45:18). The adj. calen etymologically means "bright", but is used = "green" (q.v.).
gilion
of stars
(lenited ngilion; pl. gilioen). Archaic ✱giliaun.
êl
noun. star
A Sindarin word for “star” that is largely archaic and poetic, and is mainly used as element in names like Elrond (Let/281; WJ/363; Ety/EL); the more usual word for “star” in ordinary speech was gil (RGEO/65). However, the collective form elenath is still used in common speech to refer to the entire host of stars (WJ/363). The plural of êl is elin, as this word was derived from ancient ✶elen, and the final n that was lost in the singular was preserved in the plural. In some cases Tolkien posited a restored analogical singular elen from the plural form (PE17/24, 67, 139), but this isn’t in keeping with the notion that the word was archaic, so I would ignore this for purposes of Neo-Sindarin.
Conceptual Development: This word and its root first appeared in The Etymologies of the 1930s, where N. el “star” was derived from the root ᴹ√EL of similar meaning, but was “only [used] in names” (Ety/EL). It seems Tolkien introduced the root to give a new etymology for names like N. Elrond and N. Elwing, which initially appeared under the root ᴹ√ƷEL “sky” (Ety/ƷEL).
el
star
n. star.
elen
star
pl1. elin, pl2. elenath _n._star. Its collective plural (pl2.) designates 'the (host of all the) stars, (all) the (visible) stars of the firmament'. Q. elen, pl1. eldi, eleni. o menel aglar elenath ! lit. 'from Firmament glory of the stars !'. >> êl
elen
noun. star
glân
adjective. bright, shining white
The word is deduced from its mutated form, but it is worth mentioning that a stem GALÁN "bright", with glan "daylight" (and later "clear") as derivative, is listed in the Etymologies (not included in the published text, but see VT/45:13). Most of the words meaning "white" in the Indo-Eureopean languages come from the original notion of "brightness", e.g. Greek leukós "white" is cognate with Latin lucere "to shine", lux "light". This association of sense is also found in Gnomish, PE/11:39 (glan "clean, pure", from "bright" originally) and in Early Noldorin (PE/13:144, glann "clean"). The similarity with Welsh glan (where the vowel, incidentally, is also long, though this is concealed by Welsh orthographic convention) is also striking
êl
noun. star (little used except in verses)
êl
star
pl1. elin, pl2. elenath** ** n. star. Q. elen, pl1. eldi, eleni, pl2. elelli. >> elen
tinna
glint
(vb.) *tinna- (cited as a ”Noldorin” infinitive tinno) (i dinna, i thinnar). Noun
tinna
glint
(cited as a ”Noldorin” infinitive tinno) (i dinna, i thinnar). Noun
The usual word for “star” in Sindarin which replaced archaic/poetic êl; it originally meant “(bright) spark” (RGEO/65; VT42/11). It was derived from the root √(Ñ)GIL meaning “shine (white)” (PE17/152) or “silver glint” (MR/388; PE17/22).
Conceptual Development: This word first appeared as G. gail “a star” in the Gnomish Lexicon of the 1910s (GL/42), derived from the early root ᴱ√Gil- (GL/38). In the Early Noldorin Grammar Tolkien said ᴱN. gail meant “sign, token, heavenly body” (PE13/123); earlier in this document he gave it the gloss {“life” >>} “sign” (PE13/120 and note #6). In Early Noldorin Word-lists written somewhat later, gail again simply meant “star” (PE13/143), and it was used this way in the Nebrachar poem from around 1930 (MC/217).
In The Etymologies of the 1930s Tolkien gave it as N. geil “star” from primitive ᴹ✶gilya under the root ᴹ√GIL “shine (white or pale)” (Ety/GIL; EtyAC/GIL). Christopher Tolkien incorrectly marked geil as a plural form in The Etymologies as published in The Lost Road (LR/358), but Carl Hostetter and Patrick Wynne corrected this in their Addenda and Corrigenda to the Etymologies (VT45/15). Remnants of this 1930s derivation can be seen in the forms geil/gail in notes from the 1950s, derived from ancient ✶gilyā “silver spark” (PE17/30, 152). Mostly, however, Tolkien represented this word as gil in later writings, including in The Lord of the Rings Appendix E (LotR/1113).
Possible Etymology: The etymology of gil is complicated by Tolkien’s vacillation with the singular form gail (†geil) < ✶gilyā, where the diphthong ai is the result of a-affection of the base vowel from i to e, which after i-intrusion produced ei and this diphthong became ai as usual in final syllables and monosyllables: gilyā [ >✱geli(a)] > geil > gail. Signs of this etymology can be seen in the class plural giliath as in S. Dagor-nuin-Giliath “Battle-under-Stars” (S/106).
With this alternate derivation, the plural form would still be gîl, since the ancient plural -ī prevented a-affection, and the prefixal form likewise would have been gil- (PE17/152). However, this derivation conflicts with Tolkien’s usual presentation of the singular form as gil. In some places Tolkien gave a primitive form like ✶(ñ)gillē (PE17/23) or a Sindarin form gill (PE17/50), but I think it is best to assume primitive ✱gili- to explain singular gil and class-plural giliath. This is further supported by the Telerin name Gilitíro “Starwatcher”; hat-tip to Vyacheslav Stepanov for suggesting this (PE23/143).