astronomy cites

[Douglas G. Wilson]

>... does anyone know why "annulus" ('ring') and "annular",
>which are from the diminutive of "anus", spelled with a double "n"?
>It's not that the astronomers and geometers are or were embarrassed
>by the association, is it?  (OED just mentions an "erroneous"
>medieval spelling.)

I don't know the answer, and I don't even know whether the answer is known
with certainty.

There were apparently classical Latin words "annus" =
"circle"/"circuit"/"year", "anus" = "old woman", "anus" = "rump"/"anus"
(paraphrasing and simplifying from the Perseus Project Lewis-Short
dictionary online). The Lewis-Short dictionary asserts that "anulus" is not
a diminutive, but it looks like one. Then it might have been natural to
take "an[n]ulus" = "ring" as a diminutive of "annus" (a finger ring is like
a small circle) rather than of "anus" because

(1) "anus" = "rump" may not have been a common word in that milieu, and a
finger ring doesn't resemble a small anus or small arse anyway ... and it's
not like a little old woman either ...

or

(2) some bishop or other authority may have gotten tired of jokes about the
folks kissing his anulus ...

or

(3) something else.

-- Doug Wilson