Dante pauses to address the reader directly — in one of the most audacious apostrophes in the poem. He warns those who have followed him in a small boat to turn back now and return to shore. The sea he is sailing is unlike any other. The few who have stretched up toward this bread of angels — the food of understanding the divine — can follow; the others should not try. The image of angelic bread is precise: the intellect's food in Heaven is truth, the truth of God's nature, and to eat it without preparation is not nourishment but confusion.
They enter the first sphere, the Moon, as light passes through water without disturbing it — a miracle of interpenetration that Dante cannot explain but simply experiences. His body enters the Moon's substance the way divine nature enters Christ's. He is inside a luminous pearl.
The theological question he puts to Beatrice is practical: what causes the dark patches on the moon's surface? He used to think it was variations in density. Beatrice, gently, corrects him. If density alone caused the dark spots, then in a solar eclipse the light would pass through the less dense patches — but it doesn't. The real answer, she explains, is that the divine light descends through the spheres and is differentiated not by material density but by the varying capacity of the spheres to receive and transmit it. Virtue descends from God's unity and is distributed diversely through the heavenly bodies, like joy through a living face where eyes and smile differ in expression though the soul behind them is one. The theological principle matters: what makes one heaven shine more than another is not matter but the degree of participation in divine goodness.