I know what the word "lunatic" means and it has something to do with the "Moon" as the "Online Etymology Dictionary" explains:
late 13c., "affected with periodic insanity, dependent on the changes
of the moon," from Old French lunatique, lunage "insane," or directly
from Late Latin lunaticus "moon-struck," from Latin luna "moon" (see
Luna). Compare Old English monseoc "lunatic," literally "moon-sick;"
Middle High German lune "humor, temper, mood, whim, fancy" (German
Laune), from Latin luna. Compare also New Testament Greek seleniazomai
"be epileptic," from selene "moon." Lunatic fringe (1913) apparently
was coined by U.S. politician Theodore Roosevelt.
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"Lunar" and "Lunatic" seem to have evolved in a different way. Was there any etymological reason why they had to use "lunatic" in place of "lunar" for "crazy"? How did "lunatic" evolve to mean "crazy"? (I could guess, but I don't exactly understand what "moon-struck" and "moon-sick" mean in the the above.)
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Does suffix "-tic" have any special function itself or was it just used to make a different adjective from "lunar" because the Moon was called "luna" in Latin? I found a list of words that ends with "-tic", but I can't find anything in common.
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If someone was called crazy just because he was affected by the Moon's cycle, is there a word that has anything to do with the Sun's cycle meaning "a bit less crazy" or "more mentally stable than lunatic", etc?
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Why does "lunatic" have a stronger connotation than crazy, insane, out of mind, etc.?
Edit: @Elian commented, "In France, lunatique means something along the lines of erratic or mercurial."
Best Answer
It was believed that epilepsy seizures were triggered by moonlight hence lunatic was used for those patients.
There is also a scientific publication titled The disease of the moon: the linguistic and pathological evolution of the English term "lunatic" from ncbi.nlm.nih.gov that the origin of the word is related to epilepsy:
You can read about the legal category of lunacy and the history of the word in psychiatry in the book The Moon and Madness (By Niall McCrae).