The final canticle opens with one of the most famous declarations in all of Italian poetry: the glory of God penetrates and illumines the whole universe, shining more in some places and less in others. Dante has been to the summit of that glory and cannot fully bring it back to words — yet here he is, trying. His invocation is to Apollo directly, not merely the Muses. This is a significant escalation: for the Inferno and Purgatorio, the Muses of poetry sufficed; for Paradise, Dante needs the god of light and inspiration himself, the divine source behind all poetic gift. He asks Apollo to fill him as Apollo once filled Marsyas when he drew him from the sheath of his body — a disturbing image of creative inspiration as total surrender of self.
Beatrice, standing in the Earthly Paradise, turns her eyes toward the sun with an intensity no mortal could sustain. Dante, following her gaze, looks at the sun as well — an act impossible on earth but possible here at the summit of the purified mountain, where the body is restored to something of its prelapsarian capability. He stares, then looks away at Beatrice, and sees in her eyes the sun reflected and magnified: she is the lens through which divine light becomes bearable. Looking back at the sun, Dante notices it is brighter than usual — the light has increased. Then he hears a harmony, and suddenly realizes he is no longer standing on the mountain. He is rising. He is ascending into Heaven. When it happened, he cannot say; the transition was imperceptible.
Beatrice explains with patient precision what is happening. The universe has a center toward which all things naturally tend — the Empyrean, God. Dante's soul, now purified by the ascent through Purgatory, naturally rises toward that center as fire naturally rises toward its sphere. His astonishment that he is ascending should be replaced by astonishment at anything that didn't rise when the obstacle of sin was removed. The natural state of the soul, properly ordered, is upward motion toward God. The fire is not a miracle; the failure to rise would have been the miracle — and the tragedy.