Long vs. short vowels in English words of Latin origin

Is there any way to determine if a vowel is short or long in English words of Latin origin? I've noticed that u is usually long in Latin words (e.g., Jupiter) but what about other vowels?

Of course tchrist is 100% correct about looking it up but I learned a rule-of-thumb in school that serves me well for the first vowel in a word:

If the first vowel is followed by a consonant and then another vowel, the first vowel is long - as is the "u" in jupiter or tune

If the first vowel is followed by more than one consonant, the first vowel is short - as is the "u" in rubber or rust

I'm sure there are many exceptions to this but it is a simple mnemonic tool that gives the speaker/reader some clue as to the pronunciation.

Yes, there is a way: you look it up in a dictionary.

What do you mean by long and short vowel, anyway? The stuff they teach schoolchildren? If so, then you really just use the “normal” “rules” for English vowels. Here are some possible sounds for u in Latin-derived words in English:

• /juː/ as in the stressed syllables cuticle, Europe, funeral, humor, purify, and urine, or in the unstressed syllable of accurate and mercury.
• /uː/ as in the stressed syllables of juniper, juror, Rubicon, and troubadour.

• /ʌ/ as in the stressed syllables of abundant, annul, culture, custom, number, jugular, pumice, and sulfur.
• /ɝ/, [ʌɹ] as in the stressed syllables of murmur, occur, recursive, urban, urgent, and Ursa Major.

• /ɚ/, [əɹ] as in the unstressed syllables of Excalibur, culture, murmur, lemur, and sulphur.
• /ə/ as in the unstressed syllables of album, asylum, bacterium, circus, focus, illustrate, proconsul, and suburbia.

There are many other possibilities besides. As you see, there really is nothing special about words that come from Latin in the list above.

Yes, a native speaker will be able to come up with something that makes sense even on a new word they haven’t seen before.

If you are talking not about English words, but rather about open and closed u in the traditional English pronunciation of Latin words, then that is perhaps something else again.