What does Juliet ask Romeo NOT to swear his love by and what is her reason?

In Shakespeare’s Romeo and Juliet, Juliet implores Romeo not to swear his love by the moon. She reasons that the moon is inconstant, ever-changing, and often obscured from sight. To her, this signifies the unreliability of oaths sworn in its name: _"O swear not by the moon, th’ inconstant moon, / That monthly changes in her circled orb, / Lest that thy love prove likewise variable.”_.