What light through yonder window breaks?
But soft, what light through yonder window breaks?
It is the east, and Juliet is the sun.
Arise, fair sun, and kill the envious moon,
Who is already sick and pale with grief
That thou, her maid, art far more fair than she.
In any Shakespeare play written more than a few years after Romeo and Juliet, these lines would be laughable; Romeo trots out some of the most clichéd fancies of the day. But the fact that an idea was tired did not necessarily mean it was presented in jest, especially in the years when formulaic sonnets were the rage.
That Juliet is fairer and more brilliant than the moon is meant to be taken as Romeo's sincere belief. When Juliet appears above, on her balcony, she appears like the sun at dawn, her light overpowering the moon's merely reflected brilliance. This is just one in a long series of metaphorical associations of Juliet with light; they begin at the masked ball [see DANCING DAYS], when Romeo exclaims that Juliet "doth teach the torches to burn bright!" (Act 1, scene 5, 44). Later, as Romeo is about to poison himself over what he thinks is Juliet's corpse, he insists twice that all light has been extinguished.