luna

Luna, lunae is first declension feminine and, unusually for an astronomical body, feminine in gender as well as in ending, against masculine sol. The word is a worn-down louksna, "the shining one", from the same root as lux and lumen.

It stands for the moonlight itself as often as the disc, and by extension for a lunar month, which is how a Roman counts time in a poem. Capitalised it is the goddess, identified with Diana. Sub luna means "in this world, beneath the moon", the sublunary sphere of medieval cosmology where everything is subject to change. And the belief that moonlight unsettles the mind is Roman too: lunaticus is their word before it is ours, giving English lunatic, lunar and lunacy.