Romeo speaks these lines in the so-called balcony scene, when, hiding in the Capulet orchard after the feast, he sees Juliet leaning out of a high window (2.1.44–64). Though it is late at night, Juliet’s surpassing beauty makes Romeo imagine that she is the sun, transforming the darkness into daylight. Romeo likewise personifies the moon, calling it “sick and pale with grief” at the fact that Juliet, the sun, is far brighter and more beautiful. Romeo then compares Juliet to the stars, claiming that she eclipses the stars as daylight overpowers a lamp her eyes alone shine so bright that they will convince the birds to sing at night as if it were day.
This quote is important because in addition to initiating one of the play’s most beautiful and famous sequences of poetry, it is a prime example of the light/dark motif that runs throughout the play. Many scenes in Romeo and Juliet are set either late at night or early in the morning, and Shakespeare often uses the contrast between night and day to explore opposing alternatives in a given situation. Here, Romeo imagines Juliet transforming darkness into light; later, after their wedding night, Juliet convinces Romeo momentarily that the daylight is actually night (so that he doesn’t yet have to leave her room).