Greek αι (/aj/) was regularly borrowed into Latin as ae (/aj/*).
In Latin, ae eventually monophthongized into /ɛː/; in Vulgar Latin/Proto-Romance, vowel length was lost and this eventually merged with /ɛ/ or /e/.
As a result, Modern French regularly renders Latin ae < AGrk αι with é /e/, as in éther < αἰθήρ as mentioned by Arnaud Fournet. (Old French ...
I think the vowel i: in present-day English comes from Middle English e: (the famous Great Vowel shift).
Middle English e: in aeon, aether, is probably based on French éon, éther, etc, which are a regular rendition of ai-.
The spelling ae is obviously a bit pedantic and directly taken from Greek.
It seems to work like this: when something takes your attention, really grabs you, you take it in. Karl Rahner in Spirit in the World, studying Thomistic epistemology, divides his work into two parts, sense and understanding. He says that sense is reciprocal, and this claim finally makes sense to me, twenty years later, after two readings, from the ...