How do you pronounce your Greek alphabet?

I'm no greek and nor do I speak greek, but I encounter enough greek letters every day in mathematics and science. Interestingly people's pronounciation of the letters seem to differ, and when questioned some don't have an opinion of what is correct and some do. One of my old physics teacher was so sure that tau should be pronounced like 'taw' not 'tao'. What I don't to get is, they sound completely and utterly different to how it is actually pronounced in Greek. For example 'tau' is pronounced 'tav'. So how did all this debate come about in the first place? I could understand that because different languages have different sets of phonemes so it is difficult to reproduce exactly those in a language you don't speak. But you are talking about HUGE differences. Another example is 'beta': some say 'bay-ta' some say 'beeta' while the Greek's say 'vita'. The famous 'pi' is correctly pronounced 'pee' not 'pai'. 'chi' is a misleading spelling but the correct pronounciation is not 'kai' but more like 'khee'. It just escapes me how we have managed to reinvent their pronounciation and not be able to agree on it! Any thoughts?

Jam it back in, in the dark.