1,326. Finally Calculating the Moon's Visibility

Hilchos Kiddush HaChodesh 16:19

If you want to know the latitude of the moon and whether it is north or south at the start of Friday night, 2 Iyar, in the Rambam’s current year, proceed as follows: we already know that the moon’s actual position at that time is 18 degrees, 36 minutes in Taurus (18, 36). At that time, the head of the moon’s orbit is at 27 degrees, 30 minutes in Virgo (27, 30). To arrive at the moon’s latitude, we subtract the position of the head from that of the moon; this gives us a latitude course 231 degrees, 6 minutes (231, 6). The minutes make no difference so, following the procedures described throughout this chapter, the angle of the course is 3 degrees, 53 minutes. That’s the moon’s latitude at the start of the night in question. Since the course is larger than 180 degrees, that means that it’s to the south.

Hilchos Kiddush HaChodesh 17:1

All the astronomy that has been reviewed over the past few chapters has been for the ultimate purpose of learning how to calculate when the moon will be visible. If you want to know the moon’s visibility for a given night, calculate the actual position of the sun and the moon, as well as the location of the head for the time of sighting the moon. Subtract the position of the sun from that of the moon. What’s left is called the first longitude.