Mandotim,

my first line was referring to German,
the second to Latin.

How we know how they did pronounce it? That's a tricky question (and not my field of knowledge), but the experts look at rhymes, transcriptions of foreign names etc. In this particular case, the question whether 'nucleus' has two or three syllables in Latin is even more easy to answer if one looks at poems (following a strict pattern of stressed and unstressed syllables) in which this word is used.

'Deus' is another word with the same ending and in this case the number of syllables is preserved in English: 'Deity' has three syllables and is not pronounced 'Dayty'.

I am not arguing that the majority pronounciation of 'nuclear' in English is wrong in any way. Each language will evolve without caring about consistency. 'Wrong' can become 'right' in a language within one lifespan. My remark was only my personal feeling based upon my aquaintance with Latin.

Wolfgang