The Kol Bo teaches: On Rosh Hashanah, when the Satan reports our sins, Hakadosh Baruch Hu says, “We can’t believe you without proof. Bring two witnesses who can confirm your claims.”
The Satan calls for the sun to testify that the Yidden transgressed the mitzvos of the Torah. The sun agrees to come. The Satan then needs a second witness, so it invites the moon to testify against the Jewish nation. The moon, however, goes into hiding, as it states (Tehillim 81:4), חגינו ליום בכסה, the moon is concealed on Rosh Hashanah. It hides because it doesn’t want to testify against the Jewish nation. Without two witnesses, the Satan's words are not accepted, and we are acquitted.
The Noda b'Yehudah (Tzlach, drush 1:6-8) asks that the Gemara (Kiddushin 66) states that one witness is sufficient to testify that someone has an obvious מום, a blemish. Why then, asks the Noda b'Yehudah, can the sun not testify alone? The sun can claim that the Jewish nation sinned with their eyes and ears, etc., and have become מומים בעלי, blemished people.
(This is because when one sins with his eyes, it's as though he is blind, and when one sins with his ears, it is as though he is deaf.)
The Noda b'Yehudah says that this is indeed a problem, and he concludes that the solution is teshuvah. This is the meaning of the pasuk (Devarim 23:12), ערב לפנות והיה, on erev Rosh Hashanah, במים ירחץ, rinse yourself with tears and with teshuvah, and then, השמש וכבוא, when the sun comes to testify, his testimony won't be accepted. For even if you haven’t yet completed the teshuvah process, your tears will wash away the blemish. And then, המחנה תוך אל יבוא, you will be permitted to return to the camp, for you will be acquitted.
