The otherworldly feel of this movement comes from the scale choices Bach used, which are as follows: Major, Harmonic minor, Melodic, Hungarian minor, Neapolitan minor, Persian, Persian natural 6, Ionian b5 and Harmonic Major.
The most interesting sequence occurs bars 5 - 9: C Major - D Melodic - D Harmonic minor - D Hungarian minor - A Neapolitan minor - E Persian - E Persian natural 6 - E Ionian b5 - E Major - E Harmonic Major - A Melodic - C Major.
This is an example of one of Bach's explorations of lesser known scales, especially Persian, Persian natural 6 and Ionian b5.
The whole sequence is simply a way to introduce an unusual change into the music before returning to the key from whence it came, but what an interesting diversion.
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