1,329. Accounting for the Discrepancy in the Moon's Position

Hilchos Kiddush HaChodesh 17:6

The reason these minutes are subtracted (in halacha 17:5) is because the moon’s actual position is not the place where the moon will be seen. There is a small discrepancy in both its longitude and its latitude. This discrepancy is called the visibility difference. The visibility difference for the moon’s longitude at the time when it will be visible must be subtracted from the longitude, as has been illustrated.

Hilchos Kiddush HaChodesh 17:7

When it comes to the visibility difference for the moon's latitude, if the latitude is to north, one must subtract the minutes of the visibility difference for the moon's latitude from the first latitude. If the latitude is to the south, we must add the minutes of the visibility difference for the moon's latitude to the first latitude. The result after adding or subtracting these minutes to or from the first latitude is called the second latitude.