Hayom Yom

When the Alter Rebbe was nine he studied geometry and astronomy. At ten he composed a calendar for fifteen years. When he was twelve years old, it happened that he lectured publicly on Rambam's laws of kidush hachodesh.[1] The preeminent Torah-scholars who were present at that time in the study-hall were utterly overwhelmed.

Compiled and arranged by the Lubavitcher Rebbe, Rabbi Menachem Mendel Schneerson, of righteous memory, in 5703 (1943) from the talks and letters of the sixth Chabad Rebbe, Rabbi Yosef Yitzchak Schneersohn, of righteous memory.

Much of this treatise is devoted to complex astronomical calculations relative to projected appearance of the new moon, etc.

בהיות אדמו"ר [אדננו מורנו ורבנו] הזקן בן תשע שנה למד חכמת ההנדסה והתכונה. בן עשר - סדר לוח על חמש עשרה שנה. בהיותו בן שנים עשר נזדמן שלמד ברבים הל'[כות] קדוש החדש להרמב"ם, ולא מצאו הגאונים, שהיו באותו מעמד, ידיהם ורגליהם בבית המדרש.