as(s)tronomy cites

[Douglas G. Wilson]

>"anus" = "old woman" has a short /a/; "anus" = "rump"/"anus" has long /a:/.
>They were as distinct to the Romans as "ass" = "donkey" and "arse" = "rump"
>are to the English.

That's what I was taught in my Latin class. But classical Latin (or even
late Roman Empire vernacular) speech is not fully relevant here, since the
English word is known only from 1562 or something like that, and the Latin
of that time was largely a written language with local variations in
pronunciation, and I think the spellings "anulus" and "annulus" may have
been present already in medieval Latin texts. Anyway, the choice between
"annus" and "anus" (each meaning something like "circle" at least
sometimes) is more pertinent; I just included the "old woman" for completeness.

-- Doug Wilson