إِحَاطَةَ الْهَالَةِ بِالْقَمَرِ
Like the surrounding of the moon by its halo.
Hālah is a halo seen around the moon. The preacher occupies the center and the listeners cluster around him, like the moon and its enclosing ring.
This simile communicates spatial arrangement, not an astronomy lesson or a claim that the man is literally luminous.