Plutarch, De Iside et Osiride 42

ἑβδόμῃ ἐπὶ δέκα τὴν Ὀσίριδος γενέσθαι τελευτὴν Αἰγύπτιοι μυθολογοῦσιν, ἐν ᾗ μάλιστα γίγνεται πληρουμένη κατάδηλος ἡ πανσέληνος. διὸ καὶ τὴν ἡμέραν ταύτην ἀντίφραξιν οἱ Πυθαγόρειοι καλοῦσι, καὶ ὅλως τὸν ἀριθμὸν τοῦτον ἀφοσιοῦνται. τοῦ γὰρ ἑξκαίδεκα τετραγώνου καὶ τοῦ ὀκτωκαίδεκα ἑτερομήκους, οἷς μόνοις ἀριθμῶν ἐπιπέδων συμβέβηκε τὰς περιμέτρους ἴσας ἔχειν τοῖς περιεχομένοις ὑπʼ αὐτῶν χωρίοις, μέσος ὁ τῶν ἑπτακαίδεκα παρεμπίπτων ἀντιφράττει καὶ διαζεύγνυσιν ἀπʼ ἀλλήλων, καὶ διαιρεῖ τὸν ἐπόγδοον λόγον εἰς ἄνισα διαστήματα τεμνόμενος. ἐτῶν δʼ ἀριθμὸν οἱ μὲν βιῶσαι τὸν Ὄσιριν οἱ δὲ βασιλεῦσαι λέγουσιν ὀκτὼ καὶ εἴκοσι· τοσαῦτα γὰρ ἔστι φῶτα τῆς σελήνης καὶ τοσαύταις ἡμέραις τὸν αὑτῆς κύκλον ἐξελίσσει. τὸ δὲ ξύλον ἐν ταῖς λεγομέναις Ὀσίριδος ταφαῖς τέμνοντες κατασκευάζουσι λάρνακα μηνοειδῆ διὰ τὸ τὴν σελήνην, ὅταν τῷ ἡλίῳ πλησιάζῃ, μηνοειδῆ γιγνομένην ἀποκρύπτεσθαι. τὸν δʼ εἰς δεκατέσσαρα μέρη τοῦ Ὀσίριδος διασπασμὸν αἰνίττονται πρὸς τὰς ἡμέρας, ἐν αἷς φθίνει μετὰ πανσέληνον ἄχρι νουμηνίας τὸ ἄστρον. ἡμέραν δέ, ἐν ᾗ φαίνεται πρῶτον ἐκφυγοῦσα τὰς αὐγὰς καὶ παρελθοῦσα τὸν ἥλιον, ἀτελὲς ἀγαθόν προσαγορεύουσιν. ὁ γὰρ Ὄσιρις ἀγαθοποιός, καὶ τοὔνομα πολλὰ φράζει, οὐχ ἥκιστα δὲ κράτος ἐνεργοῦν καὶ ἀγαθοποιὸν ὃ λέγουσι. τὸ δʼ ἕτερον ὄνομα τοῦ θεοῦ τὸν Ὄμφιν εὐεργέτην ὁ Ἑρμαῖός φησι δηλοῦν ἑρμηνευόμενον.

The Egyptians have a legend that the end of Osiris s life came on the seventeenth of the month, on which day it is quite evident to the eye that the period of the full moon is over. Because of this the Pythagoreans call this day the Barrier, and utterly abominate this number. For the number seventeen, coming in between the square sixteen and the oblong rectangle eighteen, which, as it happens, are the only plane figures that have their perimeters equal to their areas, bars them off from each other and disjoins them, and breaks up the ratio of eight to eight and an eighth by its division into unequal intervals. Some say that the years of Osiris’s life, others that the years of his reign, were twenty-eight; for that is the number of the moon’s illuminations, and in that number of days does she complete her cycle. The wood which they cut on the occasions called the burials of Osiris they fashion into a crescent-shaped coffer because of the fact that the moon, when it comes near the sun, becomes crescent-shaped and disappears from our sight. The dismemberment of Osiris into fourteen parts they refer allegorically to the days of the waning of that satellite from the time of the full moon to the new moon. And the day on which she becomes visible after escaping the solar rays and passing by the sun they style Incomplete Good; for Osiris is beneficent, and his name means many things, but, not least of all, an active and beneficent power, as they put it. The other name of the god, Omphis, Hermaeus says means benefactor when interpreted.