Luna was Rome's own moon goddess with an old temple on the Aventine. Diana was a woodland goddess who acquired the Moon later, by being matched up with Artemis.

The Romans were systematic about identifying their gods with Greek ones and it left them with two overlapping moons. Luna kept the calendar and the chariot; Diana got the poetry, the hunting and eventually the witches. The month name and the word 'lunatic' both descend from the one who lost the popularity contest.