Commentary Purg I 14-18

The word mezzo here has caused some problems.  It would seem to mean the air between the lunar sphere and earth, that is, the 'middle zone' between the first (lunar) celestial sphere and the surface of the earth.  For Dante's own words to this effect see Conv.III.ix.12.  There is also debate as to whether the word giro refers to this first heavenly sphere or to the circle of the horizon.  In the eighteenth century Lombardi (DDP Lombardi.Purg.I.13-18) proposed that the Starry Sphere was meant, but this has seemed preposterous to most, if not all, who have considered his notion.  Just about every other commentator who had dealt with the problem for 550 years had solved it by saying that Dante was referring to the sphere of the Moon.  Then, perhaps beginning with Bianchi in 1868, the choice suddenly swerved to the earth's horizon (DDP Bianchi.Purg.I.15).  In the past century and a third, some 80% of the commentators are solidly in this corner.  The main reasons for preferring the horizon of the earth to the sphere of the Moon are either that Dante did not believe that there was an atmosphere that reached as high as that sphere or that one could not make out anything at that distance if there were.