Is Moon Sighting Essential?
Question

Is it essential for the people to sight the new moon of Ramadan?
Praise be to Allah.The majority of scholars are of the view that sighting the new moon of Ramadan on the first night of the month is a communal obligation, and if all the people fail to do it then they are sinning. This is also the Hanafi view. 
Some of the jurists were of the view that moon sighting is recommended. 
It says in Majma` Al-Anhar (1/283): “It is a communal obligation for the people to seek the new moon on the evening of the twenty-ninth of Sha`ban or Ramadan, and also of Dhul Qi`dah, and it is obligatory for the ruler to enjoin the people to do that.”  
It says in Al-Fatawa Al-Hindiyyah (1/197): “It is obligatory for the people to seek the new moon on the twenty-ninth of Sha`ban at the time of sunset, and if they see it they must fast. If it is cloudy then they must complete the month as thirty days.” (See Fath Al-Qadir, 2/313)
It says in Kashshaf Al-Qina` (2/300): “It is recommended for the people on the night before the thirtieth of Sha`ban to sight the new moon of Ramadan .” 
It is recommended to sight the new moon so as to be on the safe side with regard to fasting and so as to avoid disputes. It was narrated that `Aishah (may Allah be pleased with her) said: The Prophet (peace and blessings of Allah be upon him) used to count the days in Sha`ban more carefully than at any other time, then he would fast when he sighted (the new moon) of Ramadan. “Narrated by Ad-Daraqutni with an authentic Isnad (chain of narration)) 
It was narrated from Abu Hurayrah (may Allah be pleased with him) in a Marfu` report: “Count the days of Sha`ban for the sake of Ramadan.” (Narrated by At-Tirmidhi) 
This hadith was classed as authentic by Al-Albani in Sahih At-Tirmidhi (678). 
It says in Tuhfat Al-Ahwadhi: “Ibn Hajar (may Allah have mercy on him) said: They should strive to count its days accurately, and that is by means of watching the waxing and waning of the moon, so that they will be certain  of when to expect the new moon of Ramadan and will not miss any of it.” 
In Al-Mawsu`ah Al-Fiqhiyyah (22/23) it says: 
“Sighting the new moon is something that is decreed by the fact that some acts of worship are connected to specific times. It is prescribed for the Muslims to strive in seeking it and it is more important on the night before the thirtieth of Sha`ban, in order to find out whether Ramadan has begun, and on the night before the thirtieth of Ramadan in order to find out whether the month has ended and Shawwal has begun, and on the night before the thirtieth of Dhu’l-Qa’dah to find out if Dhu’l-Hijjah has begun. These three months have to do with two of the pillars of Islam, namely fasting and Hajj, and the setting of the dates for `Eid Al-Fitr and `Eid Al-Adhha. 
The Prophet (peace and blessings of Allah be upon him) encouraged seeking the sighting of the moon . It was narrated that Abu Hurayrah (may Allah be pleased with him) said: The Prophet (peace and blessings of Allah be upon him) said: “Fast when you see it and break the fast when you see it, and if it is too cloudy then complete the number (of days) of Sha`ban as thirty.” And it was narrated from `Abdullah ibn `Umar (may Allah be pleased with him) that the Messenger of Allah (peace and blessings of Allah be upon him) said: “The month is twenty-nine days, so do not fast until you see it, and if it is cloudy then complete the month as thirty days.” The first Hadith enjoins fasting the month of Ramadan when its new moon is sighted or when the month of Sha`ban is completed with thirty days, and it enjoins ending the fast when the new moon of Shawwal is sighted, or when Ramadan is completed with thirty days. The second Hadith forbids fasting Ramadan before the new moon is sighted or before Sha`ban has been completed if it is cloudy. 
And there is a Hadith narrated from the Prophet (peace and blessings of Allah be upon him) in which he enjoined paying attention to the new moon of Sha`ban because of Ramadan. He said: “Count the days of Sha`ban carefully for the sake of Ramadan.” This Hadith shows the importance of paying attention to the month of Sha`ban in order to work out when Ramadan starts. It was narrated from `Aishah (may Allah be pleased with her) that the Prophet (peace and blessings of Allah be upon him) used to count the days in Sha`ban more carefully than at any other time, then he would fast when he sighted (the new moon) of Ramadan, and if it was cloudy he would count thirty days and then fast. 
The commentators said: i.e., he would make sure to count the days of Sha`ban carefully in order to begin the fast of Ramadan at the right time. During the lifetime of the Prophet (peace and blessings of Allah be upon him) and after he died, the Companions (may Allah be pleased with them) were concerned with sighting the new moon of Ramadan and they used to look for it. It was narrated that `Abdullah ibn `Umar (may Allah be pleased with him) said: The people sighted the new moon and I told the Messenger of Allah (peace and blessings of Allah be upon him) about that, and he fasted and told the people to fast. 
It was narrated that Anas ibn Malik (may Allah be pleased with him) said: We were with `Umar between Makkah and Madinah and we looked for the new moon. I was a man who had keen eyesight and I sighted it, but no one else claimed that he had seen it except me. I started saying to `Umar: Do you not see it? And he could not see it.  
The Hanafis regard sighting the new moon of Ramadan on the night before the thirtieth of Sha`ban as a communal obligation, and if they sight it they should fast, otherwise they should complete the month (as thirty days) and then fast, because that which is essential to performance of an obligatory duty is also obligatory. 
The Hanbalis say that it is recommended to sight the new moons so as to be on the safe side with regard to fasting and so as to avoid disputes . We did not find any clear view from the Malikis and Shafi`is on this issue.”
Shaykh Ibn `Uthaymin (may Allah have mercy on him) was asked: Are all the Muslims sinning if none of them sights the new moon at the beginning or end of Ramadan? 
He replied: 
“Sighting the new moon of Ramadan or Shawwal is something that was established at the time of the Companions (may Allah be pleased with them), because Ibn `Umar (may Allah be pleased with him) said: The people looked for the new moon and I told the Prophet (peace and blessings of Allah be upon him) that I had seen it, so he fasted and told the people to fast.  
Undoubtedly the way of the Companions (may Allah be pleased with them) is the best and most complete guidance.” (48 Su’alan fis-Sawm, no. 21) 
So it seems that sighting the new moon of Ramadan, Shawwal and Dhul-Hijjah is a communal obligation, because it is connected to two of the pillars of Islam: fasting and Hajj. 
And Allah knows best.