The Rambam · Sefer Zemanim
הלכות קידוש החודש ט״זSanctification of the New Month 16
Chapter 16 of 19 in Sanctification of the New Month — 19 halachot
Halacha 1א׳
הָעֲגֻלָה שֶׁסּוֹבֶבֶת בָּהּ הַיָּרֵחַ תָּמִיד הִיא נוֹטָה מֵעַל הָעֲגֵלָּה שֶׁסּוֹבֶבֶת בָּהּ הַשֶּׁמֶשׁ תָּמִיד. חֶצְיָהּ נוֹטֶה לְצָפוֹן וְחֶצְיָהּ נוֹטֶה לְדָרוֹם. וּשְׁתֵּי נְקֻדּוֹת יֵשׁ בָּהּ זוֹ כְּנֶגֶד זוֹ שֶׁבָּהֶן פּוֹגְעוֹת שְׁתֵּי הָעֲגֻלּוֹת זוֹ בָּזוֹ. לְפִיכָךְ כְּשֶׁיִּהְיֶה הַיָּרֵחַ בְּאַחַת מִשְּׁתֵּיהֶן נִמְצָא סוֹבֵב בְּעִגּוּלָהּ שֶׁל שֶׁמֶשׁ כְּנֶגֶד הַשֶּׁמֶשׁ בְּשָׁוֶה. וְאִם יֵצֵא הַיָּרֵחַ מֵאַחַת מִשְּׁתֵּי הַנְּקֻדּוֹת נִמְצָא מְהַלֵּךְ לִצְפוֹן הַשֶּׁמֶשׁ אוֹ לִדְרוֹמָהּ. הַנְּקֻדָּה שֶׁמִּמֶּנָּה יַתְחִיל הַיָּרֵחַ לִנְטוֹת לִצְפוֹן הַשֶּׁמֶשׁ הִיא הַנִּקְרֵאת רֹאשׁ. וְהַנְּקֻדָּה שֶׁמִּמֶּנָּה יַתְחִיל הַיָּרֵחַ לִנְטוֹת לִדְרוֹם הַשֶּׁמֶשׁ הִיא הַנִּקְרֵאת זָנָב. וּמַהֲלָךְ שָׁוֶה יֵשׁ לְזֶה הָרֹאשׁ שֶׁאֵין בּוֹ לֹא תּוֹסֶפֶת וְלֹא גֵּרָעוֹן. וְהוּא הוֹלֵךְ בַּמַּזָּלוֹת אֲחוֹרַנִּית מִטָּלֶה לְדָגִים וּמִדָּגִים לִדְלִי וְכֵן הוּא סוֹבֵב תָּמִיד:
The orbit in which the moon always veers from the orbit in which the sun always moves. Half of [the moon's] orbit veers to the north and half of it veers to the south. And there are two points in it – one across from the other – in which the two orbits intersect one another. Hence when the moon is in one of those two points, it comes out that it is moving [along] the orbit of the sun exactly opposite to the sun. But if the moon moves out of one of these two points, it comes out that it moves north of the sun or south of it. The point from which the moon starts to veer to the north of the sun is called the head. And the point from which the moon begins to veer to the south of the sun is called the tail. And there is an even movement to this head that has no addition or subtraction; and it moves backwards through the constellations, from Aries to Pisces, and Pisces to Aquarius. And likewise does it always move [regarding the other constellations].
Halacha 2ב׳
מַהֲלַךְ הָרֹאשׁ הָאֶמְצָעִי בְּיוֹם אֶחָד ג' חֲלָקִים וְי"א שְׁנִיּוֹת. נִמְצָא מַהֲלָכוֹ בְּי' יָמִים ל"א חֲלָקִים וּמ"ז שְׁנִיּוֹת. וְנִמְצָא מַהֲלָכוֹ בְּק' יוֹם ה' מַעֲלוֹת וְי"ז חֲלָקִים וּמ"ג שְׁנִיּוֹת. סִימָנָם הי"ז מ"ג. וְנִמְצָא מַהֲלָכוֹ בְּאֶלֶף יוֹם נ"ב מַעֲלוֹת וְנ"ז חֲלָקִים וְי' שְׁנִיּוֹת. סִימָנָם נ"ב נז"י. וְנִמְצָא שְׁאֵרִית מַהֲלָכוֹ בַּעֲשֶׂרֶת אֲלָפִים יוֹם קס"ט מַעֲלוֹת וְל"א חֲלָקִים וּמ' שְׁנִיּוֹת. סִימָנָם קס"ט לא"מ. וְנִמְצָא מַהֲלָכוֹ לְכ"ט יוֹם מַעֲלָה אַחַת וְל"ב חֲלָקִים וְט' שְׁנִיּוֹת. סִימָנָם א' לב"ט. וְנִמְצָא מַהֲלָכוֹ לְשָׁנָה סְדוּרָה י"ח מַעֲלוֹת וּמ"ד חֲלָקִים וּמ"ב שְׁנִיּוֹת. סִימָנָם י"ח מ"ד מ"ב. וְאֶמְצַע הָרֹאשׁ בִּתְחִלַּת לֵיל ה' שֶׁהוּא הָעִקָּר הָיָה ק"פ מַעֲלוֹת וְנ"ז חֲלָקִים וְכ"ח שְׁנִיּוֹת. סִימָנָם ק"פ נ"ז כ"ח:
The average movement of the head in one day is three parts and eleven seconds. It comes out that its movement in ten days is thirty-one parts and forty-seven seconds. And it comes out that its movement in a hundred days is five degrees, seventeen parts and forty-three seconds. Its symbol is 5.17.43. And it comes out that its movement in a thousand days is fifty-two degrees, fifty-seven parts and ten seconds. Its symbol is 52.57.10. And it comes out that its movement in ten thousand days is one hundred and sixty-nine degrees, thirty-one parts and forty seconds. Its symbol is 169.31.40. And it comes out that its movement in twenty-nine days is one degree, thirty-two parts and nine seconds. Its symbol is 1.32.9. And it comes out that its movement in an ordered year is eighteen degrees, forty-four parts and forty-two seconds. And the average [place] of the head at the beginning of the Night of the Fifth (Wednesday night) which was the base time is one hundred and eighty degrees, fifty-seven parts and twenty-eight seconds. Its symbol is 180. 57. 28.
Halacha 3ג׳
אִם תִּרְצֶה לֵידַע מְקוֹם הָרֹאשׁ בְּכָל עֵת שֶׁתִּרְצֶה. תּוֹצִיא אֶמְצָעָם לְאוֹתוֹ הָעֵת כְּדֶרֶךְ שֶׁתּוֹצִיא אֶמְצַע הַשֶּׁמֶשׁ וְאֶמְצַע הַיָּרֵחַ. וְתִגְרַע הָאֶמְצַע מִש"ס מַעֲלוֹת. וְהַנִּשְׁאָר הוּא מְקוֹם הָרֹאשׁ בְּאוֹתָהּ הָעֵת. וּכְנֶגְדּוֹ לְעוֹלָם יִהְיֶה מְקוֹם הַזָּנָב:
If you want to know the place of the head at any time that you want, produce their average [place] for that time in the way that you produce the average [place] of the sun and the average [place] of the moon. [Then] subtract the average from three hundred and sixty degrees; and the remainder is the place of the head at that time. And the place of the tail will always be across from it.
Mishneh Torah, Torat Emet 363 edition, via Sefaria — public domain
English: Sefaria Edition, tr. R. Francis Nataf, 2019 — CC BY