Daily Talk
The Rambam ends seventeen chapters of astronomy by quoting the Sages: the sun knows the time of its setting, and the moon does not know. He offers it as the reason his procedure for sighting the new moon carries four longitudes, a table of twelve constellations, and a rule that reverses itself halfway around the sky. The whole apparatus exists to compensate for a body that will not repeat itself. Why anchor the calendar of an entire people to the one light in the heavens that cannot be trusted to keep time?
The Moon Does Not Know
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About This Talk
Chapter 17 is the chapter every earlier calculation was built for, turning the true positions of sun and moon into a verdict on whether the new moon will be visible in Eretz Yisrael tonight. The Rambam derives the first longitude and the first latitude, warns explicitly to keep both figures at hand, gives thresholds of nine and fifteen degrees that settle the matter without further work, and then restricts those thresholds to the half of the sky from Capricorn through Gemini, the other half running on ten and twenty-four. He then builds the arc of sighting through four corrections: a sighting adjustment per constellation subtracted from the longitude and added or subtracted from the latitude, the circuit of the moon set aside as a fraction of the second latitude and applied in a direction that reverses at the boundary of the zodiac, a proportional adjustment producing the fourth longitude, and two thirds of the first latitude for geographic latitude. The chapter closes with five sighting limits that read the arc against the first longitude, a fully worked example for the second of Iyar yielding an arc of eleven degrees eleven minutes, and the statement that since these matters are proven, we rely not on the individual who taught them but on the proofs he presented.