Commentary Purg V 4-6

Singleton (DDP Singleton.Purg.V.5-6) argues that, because when Dante approached these late-repentant souls in the previous canto the sun was before him ([Purg IV 101]), his shadow now fell behind him and, for this reason (or because, as Grabher [DDP Grabher.Purg.V.1-9] noted, in the shade of the boulder he was out of the sun), was not observed by the onlookers until now when, moving away from them, he cast a shadow at an oblique angle.  The sun was to Dante's left when he turned back toward the east ([Purg IV 52-57]); now as he heads west it is to his right, casting his shadow to his left.  (For the various moments in this cantica in which Dante's shadow is remarked upon, see C.Purg.III.16-18.)