The usual name for the Moon in Sindarin, from an augmented form of the root √THIL (Let/425; Ety/THIL). This word is most likely the proper name of the Moon, analogous to English “Luna”: in The Etymologies of the 1930s Tolkien indicated Ithil was a “poetic name”. Compare this to S. Raun “Wanderer”, which is more descriptive of the nature of the body and hence closer to “Moon” (and “moon”).
Conceptual Development: The first precursor to this name seems to be G. Thilim “Moon” in an early list of names (PE14/13). This became G. {Thil >>} Sil “Rose of Silpion, Moon” (GL/67, 72) and G. {Thilmos >>} Silma in the Gnomish Lexicon of the 1910s (GL/33, 67), both derivatives of the early root ᴱ√SILI as suggested by Christopher Tolkien (LT1A/Sil). The name Ithil first appeared in early versions of the tales of Númenor from the 1930s (LR/41).
_n. Astron._Moon. Q. Isil. >> Anor, ithildin