I'm making a video which includes some information about the ancient Saxon and Norman political organization of the English county of Sussex. One thing I am stuck on is the Old English pronunciation of "Rape", which is the old term for the six political / administrative areas of Sussex. There are six rapes (see Rape (county subdivision)) in Sussex. This has nothing to do with violence, as the etymology of the word is given as having descended from the Old English word for "rope". Let's not argue about that; it seems that this has been more or less settled in linguistics at this point.
It is written "...the derivation of the word from the Old English rāp (rope) has been made practically certain." The text links "rāp" to Wiktionary and this gives the IPA pronunciation as /rɑːp/.
My problem is how is /rɑːp/ supposed to be pronounced? In the IPA pronunciation guides I've looked at online there is no character ɑː listed! Perhaps the ː isn't part of the vowel coding and means something unrelated to the vowel? I don't see this anywhere indicated, perhaps because I don't know where to look.
I'd love an explanation! I'd particularly like to know how to pronounce /rɑːp/. If the OE were supposed to be pronounced the same as the word for sexual violence, the IPA would look like /ɹeɪp/, so THAT'S out, at least.
ETA: Since there seems to be some question on this point, I am NOT asking how the word "rape" is pronounced in connection with the term for the Sussex administrative district. The contemporary pronunciation is the same as the word pertaining to "sexual assault". I am asking how the given IPA code is pronounced. It's irrelevant that this is how it was pronounced in OLD English.