If the people of the West sight it, must the people of the East fast? The answer is: No, because during the time it takes to traverse this distance, the moon may have fallen behind.
The third view is that the people follow the ruler: when he fasts, they fast, and when he declares the end of the fast, they do likewise. If there were a single caliphate encompassing all Muslims, and the people sighted it in the land of the caliph, after which the caliph ruled that the sighting was established, then everyone under his authority—whether in the easternmost or westernmost regions of the earth—would be required to fast or end the fast, so that the Ummah would not differ while under one authority, resulting in dispute and division. This is so from the standpoint of rationale.
As for the textual evidence, it is the Prophet’s saying, may Allah’s peace and blessings be upon him: “Fasting is the day when the people fast, and the end of the fast is the day when the people end their fast.” (1) Thus, the people follow the ruler, and the ruler must, according to the preponderant view, act in accordance with the differing horizons.
This is how people act today: once the matter is established before the ruler, everyone under his authority must adhere to either fasting or ending the fast. From a social perspective, this is a strong view. Even if we deem the second view correct—according to which differing horizons are taken into account—whoever holds that the matter is based on the horizons must not openly oppose what the people are following.
The fourth view is that the ruling based on a sighting applies to everyone to whom news of it can reach during that night. In reality, this resembles the established school of jurisprudence in the present age, because news can reach all regions of the world in less than a night,