Shakespeare's celestial motifs especially serve to develop characterization. In particular, the celestial motifs help to portray Romeo as a young, flighty, romantic who has his head in the clouds.
We see the first reference to a celestial motif when Romeo sees Juliet for the first time at the Capulet's ball. Romeo compares Juliet to a bright star in the night sky in saying,
O, she doth teach the torches to burn bright!
It seems she hangs upon the cheek of night
Like a rich jewel in an Ethiop's ear--. (I.v.46-48)
In referring to her as hanging on the black "cheek of night" and saying that she "burn[s] bright" he is referring to her as a bright star, because only bright stars or the moon burn brightly in black night skies. This allusion serves to characterize Romeo because only a romantic would think of comparing beauty to a celestial being. Furthermore, since Romeo thinks so frequently about the skies, we see that Romeo has his head up in the clouds; he only dwells on daydreams and other fanciful notions.
Shakespeare's recurring celestial motif can also symbolically represent the sexual allusions throughout the play. Moreover, the sexual, celestial motif serves to help characterize Romeo as a very young man who is still moved and controlled by his hormones. We see a celestial motif with sexual connotations in the famous balcony scene, Act 2, Scene 2. Romeo equates Juliet's beauty with the sun and compares her to the moon. He further tells her to cease being the moon's "maid," the term maid representing both a servant to the moon and a reference to Juliet's maidenhood. He then tells her that only "fools" wear the moon's "vestal livery" and to "cast it off" (II.ii.2-9). The word "vestal" means "virginal," while "livery" refers to a "uniform," or "characteristic" clothing (Random House Dictionary). Hence, Romeo is telling Juliet to "cast off" her clothes, or her maidenhood, under the moon. This sexual celestial reference again shows us just how young and flighty Romeo is and how well he is governed by his young hormones.
Since the role of FATE is critical in Romeo and Juliet, Shakespeare uses celestial beings as representations of FATE. The arrangement of the stars, sun and moon (as you probably associate with the study of astrology and your horoscopes) dictate events on earth. When Romeo says that he feels some "consequence yet hanging in the stars" before he goes to the fateful party where he meets Juliet, he is, in essence, acknowledging that the universe is NOT aligned in his favor that night.
Much later in the play when he is told that Juliet is dead, he shakes his fist at the heavens and proclaims, "Then I defy you, stars!" which means that he no longer will play the victim of fate, but rather, take matters into his own hands and kill himself.