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

But in this talk1 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?

1just in case the link breaks: "Skyfield and 15 Years of Bad APIs - Brandon Rhodes" PyCon Canada, August 16, 2013

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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. Commented Jan 28, 2017 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
    Commented Jan 28, 2017 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
    Commented Jan 28, 2017 at 3:15
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    @tchrist Well, they clearly didn't know their Ancient Greek stress allocation restrictions. For shame! Commented Jan 28, 2017 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… Commented Jun 14, 2018 at 0:37

3 Answers 3


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
    Commented Jan 28, 2017 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
    Commented Jan 28, 2017 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
    Commented Jun 13, 2018 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
    Commented Nov 6, 2020 at 3:51

There is a simple way to look at the dilemma. Look at what the singular ending -is. It is in Greek an adjectival form, indicating son or daughter of. The genitive singular form becomes '-idis' and the nominative plural becomes '-ides'.

So Peisistratos who became 'tyrant' in Athens 546BCE till his death in 527. The tyranny was maintained by his next two male generations till their overthrow in 510. Collectively, they were known by the clan name of 'Peisistratides (Πεισιστρατιδες)', or the 'Peisistratid clan'. One of them is called a Peisistratis (Πεισιστρατις). But the singular is not often found, because clans are generally and inevitably plural. Historians of ancient Greek history, though, use Peisistratid rather than peisistratis as the true 'base' or 'stem' word. And they use the anglo-saxon '-s' to form the plural.

Clan names come a lot into Greek tragedy, also. The daughters of King Danaus (the Danaides - Δαναιδες), in English Danaids are fleeing from forced marriage to the sons of Aegyptos and come to Argos to plead for (Hiketeuo) refuge to the king Pelasgos. The tetralogy (three tragedies and a satyr play) is called The Danaids - Δαναιδες or Daughters of Danaus. In English this translates to The Danaids, and the one surviving individual tragedy is called Hiketides - Ικετιδες. This usually comes into English as Suppliants or Suppliant Women. Plays at this time were often named for their choruses, which, in this play, consisted of the suppliant daughters of Danaus. Their natural number is plural, not singular: The are a plurality, just like members of a family.

Just as the chorus was a plurality, so, to the ancient watcher of the night sky, constellations were and are by definition pluralities, or, in other words, a sort of 'plural unit'. On the analogy of Peisistratid, one member of the ephemerides would be an ephemerid, but more commonly thought of as one of the ephemerides and one of the Pleiades would be a Pleiad but more naturally as a member of (or just one of) the Pleiades. The Cambridge English dictionary gives 'Pleiades' as the spelling.

  • Thank you for adding this wonderful and helpful answer!
    – uhoh
    Commented Oct 12, 2022 at 22:50
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    @uhoh - But I left out the pronunciation. There are two classes of ending in '-ides': one for astronomical terms like ephemerides in English. It is 'id-ez' (not '-eyedz'). But, as I hope I explained, it is quite common to omit the final 'e' and pronounce '-idz', which is the more fully anglicised version. But there is a second class of chemical terms like 'peptide'. The 'ide' part is derived from the Greek word 'eidos' ('ειδος'), meaning 'form' or 'type'. Classicists till 60 years ago mispronounced the 'ei' as 'eye' and were copied by scientists. So 'pepteyedz' it is (they are).
    – Tuffy
    Commented Oct 13, 2022 at 18:53

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