In Beethoven's Moonlight Sonata 3rd movement, I'd like to understand the Fx (double sharp) in measure 8.
Measure 8 has a bass of A and an arpeggio with the notes C# E Fx C# which sounds like an A7, but the Fx can't be the seventh, since otherwise it would have been written as a G. If it's not, it serves a specific purpose that I don't get.
With Fx, this chord would be more like an A6, but with an augmented sixth... which I think is unusual.
Is there a reason to avoid the dominant 7th by keeping the "F" name?
Putting measure 8 in context: the previous measure 7 is F#m, and the next measure 9 is G#. I'd like to think that this may be a II-V-I chord progression, and I would expect in that sense a C#m chord. A7 would be the relative major so that would fit...
Would it be the C#m I expect, but with an "augmented sus4" (that sounds like G natural...) and on top of an A bass?