“Is it not obvious?” Luna said bitterly, waiving a hoof at her coat.  “Thou art light, we are dark; thou hast the sun, we the night.”  And it was not long after that she had been forced to either kill Luna or banish her to the moon, and after a thousand years she still had not decided if she had made the correct decision, or been swayed by emotion.  Clearly, it was not a subject she was planning to ever bring up with her sister again.