Rambam - 3 Chapters a Day · Chabad.org: SIE Rambam

11th Perek: Laws of Kidush Hachodesh

April 6, 2026·38 min·3 clips

How did ancient Jewish sages calculate whether the moon would be visible on Rosh Chodesh night?

Chapter 11 sets the ground rules. The host explains that the earlier halachos already depend on exact Beis Din calculations about whether the new moon could be seen. Now the class gets into the method. Rambam assumes that someone drawn to wisdom will want to know how those hidden determinations are made, so this chapter opens the astronomy and arithmetic needed for what comes next. The circle of stars comes first. It is divided into 360 degrees, starting at the western end of the first constellation, called here the ram rather than by Greek astrological names. Each degree is broken down again. The host pauses over an easy mistake: these minutes and seconds are not clock time. They are fractions of curvature in a circle. The breakdown can keep going. Minutes become seconds, seconds become thirds, and the point is simple enough: when the calculation needs more precision, the system has room for it. The examples make the rule less abstract. A planet at 70 degrees, 30 minutes, and 40 seconds is found by counting thirty-degree constellations from the zero point. The host works it slowly. Sixty degrees cover the first two constellations, which leaves ten degrees and the added fractional measure inside the constellation of the twins. A second example checks the method: 320 degrees puts the planet twenty degrees into the eleventh constellation after three hundred degrees have been counted. The closing section turns to geography. Longitude here follows the ancient center of the populated regions of the earth, not the modern Greenwich reference point. Yerushalayim is placed twenty-four degrees west of that center, and the locations used for these calculations fall between twenty-one and twenty-seven degrees west.

As heard by us

A careful primer in lunar calculation, where halacha meets the geometry of the sky.

Chapter 11 of Rambam's Laws of Kiddush HaChodesh turns to method, laying out the astronomy and arithmetic behind the calculations that decide whether the moon will be visible on the night of Rosh Chodesh.

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Why you'd press play

If you want the math behind whether the moon will be visible on Rosh Chodesh, this is a clean entry point.

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