Salam aleikum In Nayl al-Raja (sharh of the Safina) it says: وزاد الرملي كوالده أمرا ثامنا وهو وجوبه على من عرف الهلال بحسابه أو تنجيمه وكذا من اعتقد صدقهما Al-Ramli, and his father, added a eighth factor (to determine when Ramadan starts): its obligatory for a person who calculates (munajjim and haasib) the birth of the moon to fast and it’s also obligatory upon anyone who believes that their calculations are correct. (sorry for the translation) In the hashiya of al-Nihaya its mentioned that al-Shihab al-Ramli was asked if this can be applied to a situation when the astronomer is certain that the moon is born and that it’s possible to see it with the eye, or if it applies to a situation when the moon is born but can’t be seen or when its born and the astronomer is not sure if it can be seen or not. He answered that it applies to all three situations. But al-Ramli mentions that calculations can’t be used to force everyone in a country to fast. It applies only on a personal level, not on societal level.