The natural interpretation of the phrase in chulin,daf 60b,chatas lashem, seems to be chatas for Hash-m, for His having diminished the moon. Yet this is a puzzling statement. What can "bringing an atonement for Hash-m" possibly mean? How can Hash-m do an avera?
Daniel Pava, Hasmonaim
The consolation that the moon wanted was that it should be the cause of a Korban that was brought by Bnei Yisrael. In this fashion, it would be the direct cause of a Mitzvah done by Jews who are Ba'alei Bechirah (people with free will), a unique merit for any angel or celestial body. This gave it reason to be consoled.
Accordingly, the statement "bring for Me an atonement" does not mean that Hash-m sinned, but rather that the moon was appeased with this special gift due to the fact that it was indeed inappropriate for Hash-m to be served by two equal bodies of light. "For Me..." can be interpreted as meaning "due to the argument for My honor made by the moon, it deserves to be the cause of a Korban brought by the Jews each month."
All the best,
Yaakov Montrose