Maimonides, who work on Kiddush ha-hodesh is the subject of Yirat Shamayim, did not compose a systematic treatise on astronomy, but his competence in the subject is well illustrated by a number of passages in the Moreh Nevukhim (Guide of the Perplexed) and by his treatise on the calendar, The Sanctification of the New Moon (Kiddush Rosh Hodesh in Mishneh Torah). In the Guide there are references to technical aspects of Ptolemaic astronomy, and it is revealed that Maimonides' disciple Joseph ibn Sham$un had studied Ptolemy's Almagest under him. Maimonides states that he was acquainted with the son of Jabir ibn Aflah of Seville (d. c. 1150), the author of a well-known astronomical text which takes exception to some Ptolemaic principles. He also refers to a lost work of Ibn Baja (d. 1139), concerning the principles of astronomy, that he had obviously read with care. According to Maimonides, the physical difficulties of eccentric and epicyclic spheres need not concern the astronomer, whose task is merely to propose a theory in which the motions of the planets and the stars are uniform and circular, and conform to observation. In the Sanctification, Maimonides describes the calendric rules that were used in the time of the Sanhedrin, the rules of the fixed calendar that apply to this day, and the astronomical determination of the beginning of the month. The third section again shows Maimonides to be competent in the technical aspects of Ptolemaic astronomy, although he made no original contribution to the subject. In 1194 Maimonides wrote a letter addressed to the rabbis of southern France strongly denouncing astrology as a pseudoscience opposed to the true science of astronomy, an opinion rarely expressed by Jewish scholars in the Middle Ages. In this letter Maimonides stated that astrology was the first secular subject he studied, and that he had read everything available in Arabic on the discipline.

הסכמת רבני ברלין: ר' מאיר ב"ר שמחה ווייל, ר' שמעי' לאנדסבורג מברלין ור' ליזר סגל הורוויץ, יז אדר תקע"ז. - חלק א חלק א: (מבוא וחמשה מאמרים... במאמר ראשון דברתי מדברים הנצרכים לחשבון עברונות, במאמר שני... להתיר כמה שאלות בענין זה. במאמר שלישי חזרתי על כל האמור... בקיצור... במאמר רביעי הצבתי כמה משלים מהדברים וחשבונות השייכים לעברונות להבין מתוכן דברי הרמב"ם... בסוף קדוש החדש, ובמאמר חמישי ביארתי הלוחות שמובאין בספר). [2], מה, [2] דף. חלק ב: קראתיו כבוד אלהים (כתבתי איזה דברים על גפ"ת). יב דף. חלק ב: נדפס ונכרך יחד עם לוחות העיבור מאת רפאל הלוי מהאנאבר.