In Romeo and Juliet, Act 1, Scene 2, Romeo says
The play is full of symbolism regarding the sun and the moon; this is related in part to the "star-crossed lovers" reference in the prologue, and suggests the cosmic fate of Romeo and Juliet's love. But perhaps more significantly, the lovers refer to one another with these symbols: Juliet is the sun to Romeo "It is the East, and Juliet is the sun; arise, fair sun, and kill the envious moon," a harbinger of her part in his fate.
Juliet uses language describing and referring to the moon to address Romeo as well "Oh, swear not by the moon, th'inconstant moon, that monthly changes in her circled orb."
Referring to one another as the two celestial bodies most often associated with the male and the female, but inverted (that is, the sun is usually considered male, associated with solar gods like Apollo, and the moon feminine, associated with Artemis/Diana) offers an intriguing representation of these lovers whose story is nearly cosmological in its themes. Shakespeare could not have known this classic tale of romance would resonate so many years into the future, and it is partly these themes that have made that possible.