O, swear not by the moon, the inconstant moon,
That monthly changes in her circled orb,
Lest that thy love prove likewise variable.
– William Shakespeare
Romeo and Juliet, Act 2, Scene 2. As Romeo begins swearing his love for Juliet by the moon shining down on them, she stops him. She doesn’t want him to swear by the moon, because it constantly changes appearance during its monthly trip around the earth. Juliet doesn’t want Romeo’s love to change.