Words related to *leuk-
1590s, "situated under the moon," hence "earthly, mundane" (old cosmology), from Modern Latin sublunaris, from sub "under, beneath" (see sub-) + lunaris (see lunar). It owes its special sense to the old cosmology of heavenly spheres and ultimately to Aristotle:
The treatise On the Heavens sets forth a pleasant and simple theory. Things below the moon are subject to generation and decay; from the moon upwards, everything is ungenerated and indestructible. [Bertrand Russell, "A History of Western Philosophy"]
1650s, "brightness," from French lucidité, from Late Latin luciditas, from Latin lucidus "light, bright, clear," from lucere "to shine," from PIE *louk-eyo-, suffixed (iterative) form of root *leuk- "light, brightness." Meaning "intellectual clarity, transparency of expression" is by 1851.
moderate-sized wildcat with a short tail, penciled ears, more or less spotted fur, and 28 teeth, inhabiting Eurasia, Africa, and North America; mid-14c., from Latin lynx (source of Spanish, Portuguese, Italian lince), from Greek lyngx, an old name of the lynx found also in Armenian, Germanic, and Balto-Slavic, though often transformed or altered. Often linked to PIE root *leuk- "light, brightness," in reference to its gleaming eyes or its ability to see in the dark, but there are phonetic problems with that and Beekes suggests a loan from a non-IE substrate language.
If that men hadden eyghen of a beeste that highte lynx, so that the lokynge of folk myghte percen thurw the thynges that withstonden it. [Chaucer's "Boethius," c. 1380]
Cognates probably are Lithuanian lūšis "lynx," Old High German luhs, German luchs, Old English lox, Dutch los, Swedish lo, Armenian lusanunk'. The dim northern constellation was added in 1687 by Johannes Hevelius. Lyncean "pertaining to a lynx" (from Greek lynkeios) is attested from 1630s.
"shining by night," as the eyes of a cat, glow-worms, decaying wood, or certain high clouds seen in northern latitudes, 1812, from Latin noct-, stem of nox "night" (see noct-) + lucentem (nominative lucens), present participle of lucere "to shine, glow, be bright," from PIE root *leuk- "light, brightness." The word was originally used of tiny sea creatures, by contemporary observers reported to be nereids, but that identification is now considered doubtful.
These are the animals that illuminate the sea, like glow-worms, but with brighter splendor. I have at night, in rowing, seen the whole element as if on fire round me; every oar spangled with them; and the water burnt with more than ordinary brightness. I have taken up some of the water in a bucket, seen them for a short space illuminate it; but when I came to search for them their extreme smallness eluded my examination. [Thomas Pennant, "British Zoology," London, 1812]
Also noctilucous (1774). Noctilucid is rarely used, but could be valuable if it meant "only making sense at night." Related: Noctilucence.