When Romeo refers to Rosaline as the moon and Juliet as the sun, he means to suggest that Rosaline is pale and distant, while Juliet is bright and close. Rosaline is someone with whom Romeo is only infatuated, whereas Juliet is the one he truly loves passionately. The comparison of Juliet to the sun further emphasizes her beauty and radiance, while the comparison of Rosaline to the moon suggests that she has only a pale reflection of that beauty. These comparisons serve to highlight the intensity of Romeo's feelings for Juliet while downplaying his previous infatuation with Rosaline.
