1,327. When the Moon Definitely Will or Definitely Won't Be Visible

Hilchos Kiddush HaChodesh 17:2

After determining the positions of the head and the moon, one can calculate the moon’s latitude and whether it is to the north or the south. This is called the first latitude. The first longitude and first latitude will be important so keep these numbers handy.

Hilchos Kiddush HaChodesh 17:3

If the number calculated as the first longitude is nine degrees or smaller, it will be impossible for the moon to be seen anywhere in Israel on that night so further calculations are unnecessary. If it is greater than 15 degrees, the moon will definitely be seen throughout Israel so further calculations are again unnecessary. When the first longitude is between nine and 15 degrees, then it is necessary to make further calculations in order to determine whether or not the moon will be visible.