The boundaries of the region whose inhabitants are required to fast based on the sighting of the crescent moon

Question:
A questioner asks: What are the limits of the region that obligates those who saw the crescent moon to make their people fast, and what if the countries are large?

Answer:
The people of expertise know this; the people of calculation know the regions and the differences in moonrise times in them. The agreement of the people of knowledge, as Shaykh al-Islam said. The people of knowledge means the people of calculation. They know by timing. They know by timing the rising of the sun in a region and its setting, and the rising of the crescent moon, and so on.
Mu'awiyah and the Muslims in Sham fasted at the beginning of the month in Sham. A man came to Madinah and informed Ibn 'Abbas that Mu'awiyah and the Muslims had fasted and that the month had been confirmed for them. He said, "No, but we will not fast until we see it." Ibn 'Abbas was in Madinah and said, "We will not fast until we see it," and he did not act upon the sighting of the people of Sham. This indicates that a difference in the horizon of the moon's sighting affects fasting, and this is part of Allah's (Mighty and Majestic) facilitation for His servants . Yes.

The presenter:
May Allah reward you,