Why are the Muslims not united in their fasting?
Question

Why are the Muslims not united in their fasting even though there is only one new moon for Ramadan? In the past there was the excuse of there being no media or means of communication.
Praise be to Allah.Firstly: 
The most likely reason for the differences in the start of the fast from one country to another is the difference in sighting the new moon. Such differences are well known and it makes sense that there are such differences. 
Based on this, it is not possible to expect all the Muslims to start fasting at the same time, because this would mean that some of them were starting to fast before the new moon had been sighted and even before it had appeared. 
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) was asked about those who call for the ummah to be united in fasting and for the moon sighting to be based on its sighting in Makkah. He said: 
This is impossible from an astronomical point of view, because the sighting of the new moon, as Shaykh al-Islam Ibn Taymiyah said, differs, according to the scientists who are well-versed in this field. Because it differs, then each country should have its own ruling, according to the reports and according to science. 
The evidence from reports is the verse in which Allah says (interpretation of the meaning): 
“So whoever of you sights (the crescent on the first night of) the month (of Ramadan i.e. is present at his home), he must observe Sawm (fasts) that month”
[al-Baqarah 2:185]
If it so happens that people in a remote region of the world do not see the new moon whereas the people of Makkah do see it, then how can the words of this verse apply to those who have not seen the new moon? The Prophet (peace and blessings of Allah be upon him) said: “Fast when you see it and stop fasting when you see it.” (Agreed upon). So if the people of Makkah, for example, see it, then how can we expect the people of Pakistan and countries further east to start fasting, when we know that that the new moon has not yet appeared in their region, and the Prophet (peace and blessings of Allah be upon him) connected the start of fasting to the sighting of the moon?
 The scientific evidence is the correct analogy which we cannot contradict. We know that dawn appears in eastern regions of the earth before it appears in western regions, so if dawn has appeared in eastern regions, do we have to stop eating even though it is still night where we are? The answer is no. If the sun has set in eastern regions but it is still day where we are, is it permissible for us to break our fast? The answer is no. And the new moon is exactly like the sun, except that the timing of the new moon is monthly and the timing of the sun is daily.  The One Who said (interpretation of the meaning): 
“and eat and drink until the white thread (light) of dawn appears to you distinct from the black thread (darkness of night), then complete your Sawm (fast) till the nightfall”
[al-Baqarah 2:187] is also the One Who said (interpretation of the meaning): 
“So whoever of you sights (the crescent on the first night of) the month (of Ramadan i.e. is present at his home), he must observe Sawm (fasts) that month”
[al-Baqarah 2:185]
So the evidence of both the texts and science indicates that we should establish a separate ruling for each place when it comes to starting and ending the fast, and this should be connected to the physical sign which Allah has described in His Book and which His Prophet Muhammad (peace and blessings of Allah be upon him) established in his Sunnah, namely the sighting of the moon and the sighting of the sun or dawn. 
End quote from Fatawa Arkaan al-Islam, p. 451. 
And he said, explaining this analogy and supporting the argument of those who say that there should be different moon sightings: 
They say that the monthly timings should be like the daily sightings. Just as different countries vary in the start and end of the fast each day, so too they must differ in the start and end of the month-long fast. The difference in daily timings is well known according to Muslim consensus; those who are in the east start fasting before those who are in the west, and they also break the fast first. 
If we accept the differences in sighting with regard to daily timings, then we should also accept it with regard to the month. 
No one can say that the verse “and eat and drink until the white thread (light) of dawn appears to you distinct from the black thread (darkness of night), then complete your Sawm (fast) till the nightfall” and the words of the Prophet (peace and blessings of Allah be upon him) “When the night has come from here and the day has departed from here and the sun has set, then the faster may break his fast” are general in meaning and apply to all the Muslims in every region. 
The same applies to the verse “So whoever of you sights (the crescent on the first night of) the month (of Ramadan i.e. is present at his home), he must observe Sawm (fasts) that month” and the words of the Prophet (peace and blessings of Allah be upon him) “When you see it fast and when you see it stop fasting.”
As you see, this opinion is very strong, and the analogy is sound, the analogy between the monthly timing and the daily timing. 
End quote from Fatawa Ramadan, compiled by Ashraf ‘Abd al-Maqsood, p. 104 
The Council of Senior Scholars issued an important statement on this topic, the text of which is as follows: 
Firstly: The difference in moon sighting is something which is well known, and there is no difference among the scholars concerning this. Rather the difference of scholarly opinion has to do with whether the difference in moon sighting matters or not. 
Secondly: The issue of whether the difference in moon sighting matters or not is a theoretical matter in which there is room for ijtihaad. Even people of great knowledge and piety differed concerning this matter. This is a type of difference which is acceptable, where the one who makes ijtihaad and gets it right will have two rewards, one for his ijtihaad and the other for getting it right, and the one who gets it wrong will be rewarded for his ijtihaad. 
The scholars differed concerning this matter and there are two points of view. One is that the difference in moon sighting matters and the other is that it does not matter. Each group quotes evidence from the Qur’aan and Sunnah, and sometimes they quote the same text, such as when they both quote the verse (interpretation of the meaning): 
“They ask you (O Muhammad) about the new moons. Say: These are signs to mark fixed periods of time for mankind and for the pilgrimage” [al-Baqarah 2:189]
and the words of the Prophet (peace and blessings of Allah be upon him), “Fast when you see it and stop fasting when you see it.”
 That is because of different understandings of the texts, and different ways in which each group derives evidence from them. 
Based on the considerations that the Council has seen and examined, and based on the fact that the difference of opinion on this matter does not have any effect that may lead to undesirable consequences, since this religion appeared fourteen centuries ago and we do not know of any period during which the ummah was united in moon sighting, the members of the Council of Senior Scholars think that matters should be left as they are and that this subject should not be stirred up. Each Islamic state should have the right to choose whichever opinion it wishes, based on the suggestions of its own scholars, because each view has its evidence and proofs. 
Thirdly: The Council has studied the issue of proving the new moon by means of calculation, and what has been narrated in the Qur’aan and Sunnah, and they have studied the comments of the scholars on this matter. They have decided unanimously that astronomical calculations carry no weight in determining the new moon with regard to Islamic matters, because the Prophet (peace and blessings of Allah be upon him) said, “Fast when you see it and stop fasting when you see it.” And he (peace and blessings of Allah be upon him) said: “Do not fast until you see it, and do not stop fasting until you see it.” And because of other evidence to that effect. 
End quote, from Fatawa al-Lajnah al-Daa’imah, 10/102.