I've always pronounced "Ephemerides" (plural of Ephemeris) using four syllables, the fourth being "Ides" as in the "Ides of March".

But in this talk at a few seconds after 10:10, it is pronounced with five syllables, the last two sounding like "Pleiades."

Are both acceptable, are there rules or guidelines for choosing a preferred pronunciation between the two?

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    Pleiades has two common pronunciations, neither of whose final syllables sound like those in ephimerides to me. Could you include a phonetic transcription to make it quite clear what exactly the pronunciations you're asking about are? FWIW, I've only ever heard ephimerides (like all other Greek -is/-ides words) pronounced as Wiktionary gives it, with final /ɪdiːz/ and antepaenultimate stress. Jan 28 '17 at 2:40
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    @JanusBahsJacquet: I assume uhoh has the weak vowel merger and is saying both sound like they end in /ədiːz/.
    – herisson
    Jan 28 '17 at 2:47
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    @JanusBahsJacquet: Well, for some "is-ides" words that had a long iota in Greek, there is the option of stressing that in the plural. For example, the OED mentions /æpˈsaɪdiːz/ ("ap-SIDE-eez") for apsides. But it says /ˈæpsɪdiːz/ is also "common", and that would be more consistent with the usual pronunciation of other similar words such as "matrices", "dominatrices" etc.
    – herisson
    Jan 28 '17 at 3:15
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    @tchrist Well, they clearly didn't know their Ancient Greek stress allocation restrictions. For shame! Jan 28 '17 at 3:23
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    If you're asking whether that should be something like rides or ride-ease both my astrology teachers and used ride-ease and all their coupole of dozen students were already using ride-ease before signing up… Jun 14 '18 at 0:37

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.

  • The octopodes post is very helpful, as are other answers there as well. I'll work with a linguist later today to help me with the pronunciation notation. Thank you for the quick response!
    – uhoh
    Jan 28 '17 at 3:00
  • Yes! I now see that "Pleiades" is was a poor choice. While the induced flashback to second grade caught me off-guard, this is perfect. Thank you!
    – uhoh
    Jan 28 '17 at 3:22
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    Noting that 'ephemeral' is pronounced /e 'fem ə rə/ and 'hyperbole' /hai 'pɚ bə li:/ and 'sequipedalian' /se skwi pə 'dei li ən/, you said all that without remarking pedantically that Greek borrowings into English usually put stress on the antepenultimate. I mean where else can you use that word?
    – Mitch
    Jun 13 '18 at 21:26

As a supplement to @herisson's excellent answer, the following three online dictionaries provide an audio link for the pronunciation of the plural noun. The audio is embedded in each web page so I can't insert the audio links here, but I've included a link to the page and an edited excerpt of the dictionary entry in each case:


ephem·​er·​is | \ i-ˈfe-mə-rəs \
plural ephemerides \ ˌe-​fə-​ˈmer-​ə-​ˌdēz \


ephemeris (ɪˈfɛmərɪs)
plural ephemerides (ˌɛfɪˈmɛrɪˌdiːz)


ephemeris [ ih-fem-er-is ] [IPA: / ɪˈfɛm ər ɪs /]
plural e·phe·mer·i·des /ˌɛf əˈmɛr ɪˌdiz/.

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    Excellent, thanks! The difference in mɛr between the first two is pronounced (pardon the intended pun)
    – uhoh
    Nov 6 '20 at 3:51

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