In Act 2, scene 2, Romeo uses a metaphor that compares Juliet to the sun. Standing under her balcony, he says,

But soft, what light through yonder window breaks?
It is the East, and Juliet is the sun. (2.2.1-2)

He insists that the moon is envious of her because Juliet is so beautiful, more beautiful even than the moon. However, later, after the couple has been secretly married but before Juliet learns that Romeo has killed her cousin, Tybalt, she thinks of her new husband. In regard to him, she says,

[...] when I shall die,
Take him and cut him out in little stars,
And he will make the face of heaven so fine
That all the world will be in love with night
And pay no worship to the garish sun. (3.2.23-27)

To me, this always seems to signify that their love is doomed and that they will not be able to be together. Juliet associates Romeo with the beauty of the stars at night, calling the sun gaudy. However, Romeo says that the moon is jealous of Juliet, who is bright and vital like the sun. They literally get to spend this one day and one night together before Romeo goes into exile. The next morning, Juliet tries to convince him to stay, telling him that it is the nightingale "sings on yond pomegranate tree," not the lark (a bird of the morning) (3.5.4). She does not want it to be day because that means Romeo will have to leave her.

In the end, Romeo's association of Juliet with the sun and Juliet's association of Romeo with the stars seem to foreshadow the fact that they will not be able to be together for very long: only for a very short time—just like the sun and stars are not visible to us simultaneously.

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.