For the source text click/tap here: Shavuot 9
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The Mishna (2a) taught that when a person who was tameh was unaware of his status and he entered the Temple or ate consecrated food, since he cannot bring a sacrifice for atonement (given that he was unaware that he had done anything wrong), sacrifices brought on holidays and on Rosh Ḥodesh (the New Moon) serve to offer him that atonement.
The source for this is the passage (Bamidbar 28:15) that describes how the sacrifice is a sin offering la-HaShem – to God – which is understood to refer to sins that only God is aware of.
The Gemara quotes another teaching derived from this passage. Reish Lakish says that the sacrifice brought on Rosh Ḥodesh makes reference to a sin offering la-HaShem because God says to the Jewish people that this sacrifice should be brought to atone for God having minimized the moon.
The idea that God made the moon smaller is discussed at length in the Gemara Ḥullin (60b) where the Gemara describes that originally both the light of the day – the sun – and the light of the night – the moon – were the same size, but that God made the moon smaller after it complained that two equal rulers could not exist together (see also Rashi on Bereshit 1:16).
see daf ditty chullin 60 for more.
We explore the cabalistic dimension of this with the work of Elliot Wolfson.