The PIE words for sun and moon were *soh 2 wl and *méh 1 ns, and there were also derivatives from *kand "shine" and *lewk "bright, light". The Proto-Semitic words for sun and moon were *šamš- (whence Akkadian šamšu, Hebrew šemeš, and Arabic šamš) and *warihh- (whence Akkadian warhhu and Hebrew yâréach); the Arabic word for "moon" (qámar) is from a different root. The Proto-Semitic pantheon included, separate from *Ilu (the sky god), the sun goddess *Šamšu and the moon god *Warihhu. The Akkadian name for the moon god was Sîn. None of these use "el" or "la" as names for the sun and moon.
Semetic root: l.v.h. or l.a.h. or l.v.h.a. = shine, lighten, see carefully and clearly.
Greek root: λυκ (=light) > λυ'χνος, λυχνι'α, Λυκι'α, Λυ'καστος, Λυ'κτος, Λυκει'ος, Λυκι'ων, αμφιλυ'κη, λευ'σσω, λευκο'ς (=white), Λουκα'ς (=Luke)
NOTE: [λυκο'φως* (= twilight ), λυκαυγε'ς, λυκα'βας] DO NOT COME FROM THE SAME ROOT AS ERRONEOUSLY MENTIONED IN DICTIONARIES.
* λυκο'φως IS NOT λυκ + φως. λυκ= light φως=light. light + light ?! Even a 6th grader could understand this!
Latin: lux, luceo, luna, lumen
German: lioth, light
Slavic: luca, luci
Sanskrit: ruk, roke
NOTE: I had to use apostr. for accents as accented Greek letters do not appear correct (at least in Firefox which I am using).