Commentary Par II 142-144

The greater or lesser effulgence of a star results from the conjoined qualities that a particular star has in conjunction with the informing virtues, or powers, of its angelic informant.  And this is the answer to Dante's quandary.  He had attempted to analyze the moonspots in physical terms; Beatrice has just accounted for them in metaphysical ones (notably poetic though these are, comparing the relative brightness of/in a heavenly body to the relative brightness in a joyful eye, an ocular smile, as it were).