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The moon is renewed each month. At the end of the lunar cycle, the moon becomes invisible, but only a few days pass – and we see the new moon sickle in the sky. The monthly rebirth of the moon is a sign to the people of Israel that their radiance will be renewed.
From the Rebbe’s explanation:
Source: interactive multimedia class based on the Rebbe’s sicha (Shabbat Parshat Nitzavim 5751, Rosh HaShanah and Shabbat Parshat Vayeilech, 5752).