Don't Tell Your Greek Class

I learned something fascinating about the Greek of Revelation that I had not expected, and certainly was never told. What everyone knows is that certain prepositions (such as δια, εις, υπο) take certain cases, with a number of prepositions being able to take two or three different cases, often with a semantic difference between these. The case of the substantive that follows the preposition is determined by the preposition.

But not (always) so in Revelation …

I was looking at Revelation 4:9 where the text reads τῷ καθημένῳ ἐπὶ τῷ θρόνῳ with the variant ἐπὶ τοῦ θρόνοῦ. The preposition ἐπί can take of course the genitive, dative, and accusative. But why the textual variant here? Is there part of the tradition that doesn't like the dative anymore (which is probably true)? When I started to look a little further I found something that has probably been known since John wrote Revelation (at least it is in Bousset's and Charles's commentaries on Revelation according to Josef Schmid, and they go a long way back), and that is this:

In the construction 'he who sits on the throne' the case of the prepositional phrase 'on the throne' (ἐπί + article + θρόνος) that follows the participle 'he who sits' is normally identical to the case of the participle.

So we have ἐπὶ τοὺς θρόνους ... καθημένους (4:4); τῷ καθημένῳ ἐπὶ τῷ θρόνῳ (4:9); τοῦ καθημένου ἐπὶ τοῦ θρόνου (4:10).

There are a number of exceptions. With the nominative (ὁ) καθήμενος we find both ἐπὶ τῷ θρόνῳ (21:5) and ἐπὶ τὸν θρόνον (4:2) and with other combinations of κάθημαι ἐπί (e.g. with αὐτός) it doesn't apply as much. It would be nice if someone could give a good linguistic explanation of this phenomenon.

Revelation does not contain the cleanest possible Greek grammar, but it is nice to discover that there is at least some reason behind the confusion. But how to teach this to an undergraduate class I don't know.