My Interpretation (remember that there isn’t just ONE correct interpretation)….

Juliet is quietly praying to God or quietly sharing her heart’s desires with the night sky- possibly both at the same time.  She is not speaking aloud; at the most her lips are moving without emitting any sound.  At this point she is mostly ‘speaking’ with her eyes; her eyes ‘discourse’ because they are moving from side to side as they search the sky for a sign, an answer to her prayer or an inspiration.