In this answer, fdb suggested that Greek selēnē < selannā < *selas-nā and Latin lūna < losnā < *lowks-nā share a suffix.
What is this noun-forming -nā, and is it the same one that's seen in e.g. ruina? How long did it remain productive?
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The idea is that *lowks-nā is originally a feminine (hence -ā) adjective meaning “shining”. The suffix *no commonly forms adjectives, e.g. dignus, magnus, plenus etc. It is the exact equivalent of Avestan raoxšna- “bright”.