Hilchot Kiddush HaChodesh
הלכות קידוש החודשThe Rambam's Laws of Sanctifying the New Month, from the calendar calculation through the model of the sun and moon to the arc of sighting. The Hebrew follows the Chabad text; the English is a new, plain translation set beside it, halacha by halacha.
The translation focuses on the calculation and the astronomy, so it begins at chapter 6 with the molad. Chapters 1–5, on sighting the new moon, the witnesses and the court, and adding a month to the year, can be read in Hebrew and English at Chabad.org.
Chapter 6 The molad
פרק וThe units of the calendar, the remainders of a month, a year and a nineteen-year cycle, and how to find the molad of any year or month from the first molad.
15 halachot
Chapter 7 Fixing Rosh Hashanah
פרק זThe four postponements that move Rosh Hashanah from the day of the molad of Tishrei, and why a calculation by mean motion needs them.
8 halachot
Chapter 8 Full and deficient months
פרק חWhy months are whole days, which months are full or deficient, and how to tell whether a year is complete, deficient, or in order.
10 halachot
Chapter 9 The seasons at 365¼ days
פרק טThe tekufot by the year of 365¼ days: their spacing, their hours, and the day of the month on which they fall.
8 halachot
Chapter 10 The seasons by the shorter year
פרק יThe tekufot by the year slightly under 365¼ days, which the Rambam judges the one the court relied on.
7 halachot
Chapter 11 Introduction to the astronomy
פרק יאWhy these methods are taught, the units of the circle, adding and subtracting arcs, mean and true motion, and the epoch.
17 halachot
Chapter 12 The sun's mean motion
פרק יבThe sun's daily mean motion and its apogee, their positions at the epoch, and how to find the mean sun for any day.
2 halachot
Chapter 13 The sun's true position
פרק יגThe sun's course and its correction table, worked through for the eve of 14 Tammuz.
11 halachot
Chapter 14 The moon's mean motion
פרק ידThe moon's two mean motions, their positions at the epoch, and the seasonal adjustment to the time of sighting.
6 halachot
Chapter 15 The moon's true position
פרק טוThe double distance, the corrected course and its table, worked through for the eve of 2 Iyar.
9 halachot
Chapter 16 The moon's latitude
פרק טזThe head and tail, the moon's northern and southern latitude, and its correction table.
19 halachot
Chapter 17 The arc of sighting
פרק יזFrom the first longitude to the arc of sighting, and the limits that decide whether the new moon will be seen.
24 halachot
Chapter 18 When the moon is not seen
פרק יחClouds, valleys and seasons; fixing months by calculation when the moon goes unseen; east and west of the Land of Israel.
16 halachot
Chapter 19 The moon's tilt and height
פרק יטThe declination of the zodiac, which way the new moon appears to lean, and how high it stands.
16 halachot