The lines that your question is asking about can be found in the beginning of Act 2, Scene 2. Romeo is in Capulet's orchard, and he is waiting for Juliet to come out on her balcony. When she finally does come out, Romeo compares Juliet to the sun rising in the east. As the sun (Juliet) rises, it (she) outshines the "envious moon." Romeo is saying that Juliet is so fair and lovely that she outshines any other girl that might have feelings for Romeo. The moon represents any of those other girls. The moon (another girl) is "envious," because it knows that Juliet is far brighter and more attractive than it (she) is. Scientifically speaking, the sun is definitely both brighter than the moon and more "attractive," in that its gravitational field is stronger. The sun creates its own light, and it is way more massive. More mass means more gravity. More gravity means a stronger attraction. I may not like Romeo, but his metaphor in this case is a good one.