Why are Greek letters pronounced incorrectly in scientific English?

Solution 1:

The pronunciation of Greek letters by scientists isn't very different from the pronunciation of the Greek letters in the respective countries: American scientists pronounce them pretty much the same way the general American population does, and so on.

So your question is actually about why the English pronunciation of Greek letters, and the answer is that it is based on (but not always actually very close to) the reconstruction of the Classical Greek pronunciation by Erasmus in 1528 and by John Cheke and Thomas Smith around 1540, which were adopted in schools. This pronunciation underwent some change along with the rest of English during the Great Vowel Shift, and a re-reconstruction in the mid-19th century brought it back in line (incompletely) with Ancient Greek. The Wikipedia page on Pronunciation of Ancient Greek in teaching has more details.

For sake of completeness, here's a (very incomplete) table showing the pronunciation in American English, British English, Ancient Greek, and Modern Greek. I've rearranged the alphabet to put sort-of rhyming letters together, but all letters are there.

[Disclaimer: Many of the entries may be terribly wrong. The American and British IPA entries are based on the article English pronunciation of Greek letters, the "pseudo-phonetic spellings" are from here and here. The Classical and Modern Greek pronunciation columns I made up, partly from Swedish Wikipedia, partly from piecing together each letter's pronunciation in this table, partly from here for Modern Greek, and partly on my own — and I don't actually know IPA.]

[Edit: This table has now been edited to correct the IPA and source Classical Greek pronunciations from the English Wikipedia.]

Notes:

• The table, especially the all-important Classical Greek pronunciation column, is incomplete; I ran out of patience.
• For the rhyming letters Beta-Zeta-Eta-Theta, the American pronunciation (-ayta) is closer to Classical Greek and the British pronunciation (-eeta) closer to Modern Greek.
• For the rhyming letters Phi-Chi-Psi-Xi (but not Pi for some reason!) there seems to be a variant (-ee) pronunciation close to modern Greek that exists only(?) in science and mathematics.

I've made this community wiki so that someone can fix the errors or complete the table (including possibly myself if I regain the patience to finish this sometime!)

Solution 2:

Some of the factors include:

• Ancient Greek pronunciation was, it is believed, significantly different to the modern language. How do we know? Well, that's a fascinating topic, but there are various clues, including the way animal noises were rendered (suggesting that the ancient "beta" was closer to "b" than "v", because sheep's bleating sounds more akin to "baa, baa" than "vee, vee"!). Wikipedia's article on Ancient Greek phonology has more details.
• The pronunciations have come to English via other languages (French/Latin), which have exerted their own influence.
• The pronunciations received from them went through the Great Vowel Shift, which probably explains the "pee"-versus-"pie" for "pi" aspect.

Also note that the pronunciation in English is not itself consistent: in my experience most UK speakers say "beet-a" for beta, whereas many US speakers pronounce the same letter as "bate-a".

Solution 3:

The scientific pronunciation is based on the Classical Greek pronunciation, not the Modern Greek pronunciation. In Classical Greek:

• η is [e] ("ay"), not [i] ("ee")
• υ is [y] ("yoo", approximately) or [u] ("oo"), not [i] ("ee")
• αυ is [au] (the vowel in "out"), not [av] or [af]
• β is [b], not [v]
• δ is [d], not [ð] (the "th" in "this")

The only cases where the English scientific rendition is "wrong" is for ι, which was [jota] even in Classical times, χ which is usually pronounced as [k], since the velar fricative [x] doesn't exist in English, and ψ and ξ which are simplified to [s] at the beginning of words since English doesn't allow [s] to be the second element in word-initial clusters. These represent adaptations to English phonology.