Gemini. The astrological sign is usually pronounced ‘jemmin-eye’, but the spacecraft is usually pronouced ‘jemminee’. Which is correct?
I vote “eye”. The only time I’ve heard pronounced “eee” is by southern types in describing the NASA program.
Both. It’s a personal thing - the zodiac sign can be pronounced both ways.
Your question could be asked about a number of words - often, vase, schedule etc.
I have to say that the astrological sign and the space ship are both ‘jemmin-eye’. If I were to hear ‘jemminee’ I would assume that the next word I would hear is cricket.
Yes; the astronauts seemed to prefer ‘ee’.
Or ‘Christmas’?
The space program was Geminee, always Geminee. Also, NASA’s predecessor organization was “the N.A.C.A”, not “nacka”.
I have the horrible feeling that “geminEE” is more correct, but I always says it with the “EYE” ending, jsut 'cos everyone else does.
(Hey, I’m only little,so I wouldn’t want to upset people!) 
You are correct, from the point of view of authentic Latin pronunciation. Pronouncing it with the ‘eye’ sound comes as a result of anglicized Latin pronunciation, which tends to be used by doctors, lawyers, and some scientists when Latin crops up in their professional terminology. For example, I believe biologists will pronounce a terminal -i in a species name as ‘eye’. And we call those hollow areas in our skulls “sigh-nuses” and not “seenuses”, which would be more correct Latin.
I’m kind of surprised that the NASA people used the -EE pronunciation. Of course, physicists and engineers probably have less contact with Latin than a biologist or a lawyer would, but still there were some astronomers there, or so I would think.
Hmm…I’ve only heard it pronounced with the “eye” ending, not the “ee.” Although now that it’s been pointed out to me, I’m sure I’ll start noticing this other pronunciation.
If “ee” is correct, the Moody Blues will have to re-record “Gemini Dream”. 
All 3 of the grey beards I worked with who had worked on the space program said “ee”. That said, one was from SC, another from MS, and the last from TN, so maybe it WAS a southern thing.
Yep, and the Alan Parsons Project, “Gemini.”
I have never heard it said anyway by eye.
According to my American Heritage dictionary, both are correct.