The difference between ᾰ̓́στρον (ástron) and ἀστήρ (astḗr) in Ancient Greek

Question

The words ᾰ̓́στρον (ástron) and ᾰ̓στήρ (astḗr) both apparently refer to a celestial body (typically stars and planets).

Other than ᾰ̓́στρον being a 'second declension' noun and ᾰ̓στήρ being a 'third declension' noun, and ᾰ̓στήρ having additional meanings (an illustrious person, a starfish, a songbird) I can't find any significant difference between the two.

According to Wiktionary ᾰ̓́στρον is derived from ᾰ̓στήρ, but there's nothing to indicate any usage difference between the two when used in the sense of 'star' or 'planet'.

Hence I would like to know:

• Is there any particular usage difference between the two, or are they interchangable (for the 'star'/'planet' sense)?
• If there is a difference in meaning or usage (for the 'star'/'planet' sense), what is that difference?
• Is there some reason why ᾰ̓́στρον came into existance?
  • Does it fill a role or provide a meaning that ᾰ̓στήρ does not?
  • Does it disambiguate between other meanings of ᾰ̓στήρ?
  • Is it simply an attempt to Hellenise a word of proto-Indo-European origin, or to make it more suited to a particular dialect of Ancient Greek (e.g. Attic, Ionic, Aeolic, Doric)?

(Please note that I know little about Latin or Greek grammar, so if there is some grammatical difference, e.g. if the 'second' and 'third' declension difference has some significance that I am unaware of, it would be helpful if that were explained, or if a reference to an explanation were provided.)

Answer 1

Etymologically, ἀστήρ and ἄστρον are the same word. ἀστήρ descends straightforwardly from PIE *h₂stḗr "star", and has a regular plural ἀστέρες. In PIE, besides the regular plural there was also a form called the collective, which is basically a way of referring to a mass of things that are conceived of as going together. The collective ending ended up in Greek as -α, so in addition to its regular plural ἀστήρ also at one point had a collective form ἄστρα. But -α is also the Greek neuter plural ending (because the neuter plural and the collective were originally the same thing), so ἄστρα looked to Greek speakers like a plural of a neuter noun ἄστρον, which thus sprang into existence. ἄστρον remained less common in the singular than ἀστήρ, though.

Some differences in meaning seem to have developed over time. Judging from the LSJ ("A Greek-English Lexicon") entries for ἀστήρ and ἄστρον, the latter (in the singular) was "seldom [used] of any common star" (as opposed to an important star like Sirius or the sun).

Notably Aristotle uses ἄστρον (or ἄστρα) specifically to mean "fixed star" as opposed to wandering star (i.e. "planet", which originates from the Ancient Greek πλανήτης, meaning 'wanderer'). For context: the Ancient Greeks at the time of Aristotle believed in a geocentric model of the universe whereby the "fixed stars" were affixed to a 'celestial sphere' (a sort of hollow sphere moving around the Earth at a distance), hence why they appeared to move in sync with each other, whilst the "wandering stars" ("planets") were each affixed to their own 'sphere', hence why they appeared to 'wander', unlike the 'fixed' stars.

There's an explicit discussion of the difference in one text, a Hermetic extract preserved by the fifth-century scholar Stobaeus. This says:

ἀστέρες δὲ ἄστρων διαφορὰν ἔχουσιν. ἀστέρες μὲν γάρ εἰσιν οἱ ἐν τῷ οὐρανῷ αἰωρούμενοι, ἄστρα δὲ τὰ ἐγκείμενα ἐν τῷ σώματι τοῦ οὐρανοῦ, συμφερόμενα δὲ τῷ οὐρανῷ, ἐξ ὧν δώδεκα ζῴδια προσηγορεύσαμεν.

ἀστέρες (pl. of ἀστήρ) are different from ἄστρα (pl. of ἄστρον). ἀστέρες are the ones floating/hovering in the heavens, while ἄστρα are the ones placed in the body of the heavens and carried around with it, twelve of which we call the zodiac.

This seems to be the same distinction between "fixed stars" and "wandering stars" that Aristotle implicitly draws (though the latter may be including other celestial phenomena such as comets) -- at least, Aristotle (in De Caelo book 2) uses ἄστρα in a context where he's specifically referring to the fixed stars. Whether Aristotle or other writers on astronomy make this distinction consistently, I don't know.

Answer 2

I'm sure you'll get deeper answers on this, but there seems to be no semantic difference between ἀστήρ and ἄστρον; they are both the Greek variant adaptation of the PIE *h₂stḗr; although Beekes (author of "An Etymological Dictionary of Greek") suggests the singular ἄστρον is secondary to its dominant plural ἄστρα. If I understand him right, the dominant singular is άστήρ, but its plural άστέρες defers to the above ἄστρα, which is more generic than the former.

Experts on this site might suggest if either form is more popular in specific dialects and/or times. Greek does a lot of this variation: πόλις/πτολίεθρον (pólis/ptolíethron, both meaning 'city'), etc. However, my sense is there are no subtle semantic or usage distinctions between the two variants...

I suspect nobody would flinch at Mathew 2:2 using "αὐτοῦ τὸ ἄστρον" instead of "αὐτοῦ τὸν ἀστέρα" in "εἴδομεν γὰρ αὐτοῦ τὸν ἀστέρα ἐν τῇ ἀνατολῇ καὶ ἤλθομεν προσκυνῆσαι αὐτῷ" ("For we saw his star in the east and have come to worship him.")...