How to pronounce "Ephemerides"?

All sources I've found say ephemerides is pronounced /ɛfɪˈmɛrɪdiːz/ ("effih-MERRih-deez", which for some speakers is the same as "effuh-MERRuh-deez").

Nobody uses the /aɪdz/ ("ide's") pronunciation, as far as I know. It would be justified if the singular were ephemeride ending in /aɪd/ ("ide"). However, "ephemeride" doesn't seem to exist as a singular noun.

The singular is instead ephemeris, from Ancient Greek ἐφημερῐ́ς (ephēmeris).

According to Wiktionary, the plural of ἐφημερῐ́ς was ἐφημερῐ́δες (ephēmerides). Basically, it's just become conventional to pronounce the letters "es" as /iːz/ ("eez") at the end of plural words of this type (ones that follow the Greek or Latin form rather than being formed with normal English pluralization rules).

"Ides" is a different type of loanword. It's not just a transliteration of a Greek form. The Latin word was īdūs, a plural of the fourth declension. This developed to French and English ides where the e just represents a schwa or the length of the preceding vowel, and the "s" is the usual plural suffix.