How does the sun, Juliet, change to the moon, Romeo?In other words, how does the light turn into dark. How does Juliet display that?
Romeo is speaking of Juliet in Act II, Scene ii figuratively. He is saying that he wants to experience her great beauty. He wants to see her shine. The rays of the sun often block out the appearance of the moon. He is asking her to 'shine' on him.
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,
The brightness of her cheek would shame those stars,
As daylight doth a lamp; her eyes in heaven
Would through the airy region stream so bright
That birds would sing and think it were not night.
During this entire time, Romeo is spying on her in all her beauty before she knows he is there and has time to worry or consider the consequences that they consider later.
Later, Romeo tries to swear by the moon of his passion for her. The reason he tries to swear his love is because he got to hear her thoughts and that wasn't fair - this is a dark realization. Upon noting this, Juliet doesn't want him to swear by the moon because it is inconstant.
The scene opened up with the praise of Juliet's beauty. As it continued, the reality of their circumstances became clearer and clearer, and it's not good. It shouldn't work, thus it moves from light to dark, just like the sun to the moon.