1,337. The Exacting Process to Acquire Our Data

Hilchos Kiddush HaChodesh 17:22

If we wanted to know about the arc of visibility for Friday night, 2 Iyar of the (Rambam’s) current year, we would calculate it at 11 degrees, 11 minutes, as was already discussed (in halacha 17:14). Since the arc of visibility is between 10 and 14 degrees, we must examine its relationship to the first longitude. We have already determined that the first longitude for the night in question is 11 degrees, 27 minutes. Since the first longitude is larger than 11, we see that the moon will be visible based on the visibility limits. Always follow this process to determine the relationship between the arc of visibility and the first longitude.

Hilchos Kiddush HaChodesh 17:23

The past few chapters have shown us the exacting process that was necessary to reach an approximation of the desired information without resorting to calculations that would be more complicated than necessary. All this is required in order to account for the large irregularities in the moon’s orbit. Regarding this matter, the Sages commented, “The sun knows the time it sets but the moon does not know the time it sets" (Psalms 104:19). Sometimes, its setting takes longer and sometimes it comes quicker (Rosh Hashana 25a). We see this concept reflected in these calculations - sometimes we need to add and sometimes we need to subtract in order to get the arc of visibility. As has been explained, sometimes this arc is large and sometimes it’s small.