Daily Talk
The chapter hands you two different moons, each with its own daily rate, and then tells you to correct one of them by looking somewhere else entirely. The number being fixed belongs to the moon. The number that tells you how to fix it belongs to the sun. Why should the moon's own reckoning wait on where the sun happens to be standing?
Where the Sun Stands Decides the Moon
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About This Talk
Chapter 14 gives the moon two mean rates rather than one, since the moon revolves in a small orbit that does not encompass the earth while that small orbit itself turns within a larger one that does, and it sets down the daily advance of each, the multiples for ten, a hundred, a thousand and ten thousand days, and the position each held at the fixed starting point. It then delivers the instruction that closes the chapter: before the moon's mean can be used, it must be raised or lowered by fifteen or thirty minutes, or left alone, according to which constellation the sun is standing in. What comes out is still a mean, the mean of the moon roughly twenty minutes after sunset, which the Rambam names the mean of the moon at the time of the sighting.