I’ve been trying to fill a gap in my education, never having read the entirety of the Iliad in any language (the Odyssey, yes, but not the Iliad), and now reading it in Greek. Book 2, the catalogue of ships, one commentator has this note on 484:
This struck me as both obvious and quite funny.
Hmm, where and when did the muses get their names actually? Without checking, I don’t think it was in Hesiod either - and does Hesiod even specify a number? The 9 books of Herodotus were each named after a muse, but I believe that that, along with the book division, dates from Hellenistic times.
As you guessed, it is indeed Hesiod, as they process to Olympus at the beginning of the Theogony. Such creatures typically come in threes (the Fates, the Hours, the Graces, …), and originally no doubt the Muses were three in number. Hesiod triples again, and characteristically makes up names for them.
75 ταῦτʼ ἄρα Μοῦσαι ἄειδον, Ὀλύμπια δώματʼ ἔχουσαι,
ἐννέα θυγατέρες μεγάλου Διὸς ἐκγεγαυῖαι,
Κλειώ τʼ Εὐτέρπη τε Θάλειά τε Μελπομέενη τε
Τερψιχόρη τʼ Ἐρατώ τε Πολύμνιά τʼ Οὐρανίη τε
Καλλιόπη θʼ…
The same from Diodorus Siculus (found through West’s reference on the Odyssey line):
ὁμοίως δὲ καὶ κατὰ τὸν ἀριθμὸν διαφωνοῦσιν· > οἱ μὲν γὰρ τρεῖς λέγουσιν, οἱ δ’ ἐννέα, > καὶ κεκράτηκεν ὁ τῶν ἐννέα ἀριθμὸς ὑπὸ τῶν ἐπιφανεστάτων ἀνδρῶν βεβαιούμενος, λέγω δὲ Ὁμήρου τε καὶ Ἡσιόδου καὶ τῶν ἄλλων τῶν τοιούτων. Ὅμηρος μὲν γὰρ λέγει
Μοῦσαι δ’ ἐννέα πᾶσαι ἀμειβόμεναι ὀπὶ καλῇ·
Ἡσίοδος δὲ καὶ τὰ ὀνόματα αὐτῶν ἀποφαίνεται λέγων
Κλειώ τ’ Εὐτέρπη τε Θάλειά τε Μελπομένη τε
Τερψιχόρη τ’ Ἐρατώ τε Πολύμνιά τ’ Οὐρανίη τε
Καλλιόπη θ’, ἥ σφεων προφερεστάτη ἐστὶν ἁπασέων.
West doesn’t mention this in his apparatus for the Odyssey lines, but the scholion at Iliad 24.720 says that Od 24.60-61 should be dropped:
ἀοιδούς / θρήνους: θρηνῳδούς· δεινοπενθεῖς γὰρ <οἱ> βάρβαροι. ἀθετητέος δὲ ὁ Μουσῶν ἐπ’ Ἀχιλλεῖ θρῆνος