Firstly: Sighting the Moon
1. Sighting the moon as a requirement
It is a communal obligation to seek out the moon on the thirtieth night of Ramadan. The Hanafis have explicitly mentioned this.
2. Confirming the sight of the moon
Fasting Ramadan is an obligation if the moon is sighted. Consensus has been quoted over this by Ibn Hazm, Ibn Qudamah, and al-Zarkashi.
3. The required number of witnesses of the sight
One just witness suffices for confirming the commencement of Ramadan. This is the position of the Shafi`is, Hanbalis, a group of the Salaf, and the choice of Ibn Baz and Ibn `Uthaymin.
4. The one who sights the moon by himself
Whoever sights the moon of Ramadan by himself and he did not bear witness, or he did but his account was rejected,  Benefit: whoever sights the moon and he is by himself with no other people around him, he fasts according to his sighting. then scholars have differed over what he ought to do regarding fasting according to two views:
The first: is that he fasts based on sighting it, and this is by agreement of the four schools of jurisprudence, the position of the Zahiris and the majority of scholars.
The second: is that he fasts with the masses when they fast, and does not act according to the sighting. This is the position of some of the Salaf, a narration from Ahmad, and the choice of Ibn Taymiyyah and Ibn Baz.
5. Agreement among Muslim lands or disagreement
If one land has sighted the moon, then the scholars differed over whether the rest of the Muslim lands should act according to that sighting or not. The difference is according to many views, the strongest of which are two:
The first: is that fasting becomes wajib over everyone, and this is the position of the Hanafis, Malikis, and Hanbalis.
The second: is that fasting is only wajib upon the lands who sighted it. This is the position of the Shafi`is, a group of the Salaf, and the choice of al-San`ani and Ibn `Uthaymin.
6. The ruling of relying upon satellites for the sighting
It is haram to rely upon satellites for the sighting of the moon. This is the position of Ibn `Uthaymin.
7. The ruling of using astronomical observatories for the sighting
It is permissible to use astronomical observatories to help with the sighting of the moon, like using telescopes, though it is not an obligation. If a trustworthy source sights the moon using it, then it is accepted and acted upon. This is the choice of Ibn Baz, the decision of the Permanent Committee and the International Fiqh Academy.
8. The obligation upon the captive or the prisoner
Whoever has no access to the news of the sighting and does not know the day or the month, like the captive or prisoner in the lands of war, then he must seek out the moon to the best of his ability and fast one month according to that. This is by agreement of the four schools of jurisprudence and it is the position of the majority of scholars.
9. Possible consequences of the prisoner's estimation
If the prisoner estimates  If the prisoner fasts without any use of discretion or estimation, then his fast is invalid, and he must repeat it. This is even if it was the case that his fasts matched Ramadan, since he left what is obligatory in terms of exerting and seeking the truth. This is by agreement of the jurists. a month which he fasts, then there are four possibilities in terms of outcome:
- That it becomes apparent to him that his fast matched the month of Ramadan, in which case his fast is valid, and this is by agreement of the four schools of jurisprudence.
- That it does not become apparent to him whether he matched the month or not, in which case his fasting suffices for him and he does not need to repeat. This is the position of the Shafi`is, Hanbalis, and a position among Malikis.
- That it becomes apparent to him that his fast was before Ramadan, in which case he repeats it. This is by agreement of the four schools of jurisprudence.
- That it becomes apparent to him that his fast was after the month of Ramadan, in which case it suffices him but for the days of impermissibility, i.e. the two Eids, in which case he makes up solely those days as they are invalid. This is by agreement of the four schools of jurisprudence.
Secondly: Completing Thirty Days of Sha`ban
1. If sighting of the moon is not confirmed
If sighting the moon of Ramadan is not confirmed on the thirtieth of Sha`ban, then Sha`ban becomes thirty full days. This is regardless whether the sky is clear  Mushyah refers to the sky if it is clear. or cloudy, and is by agreement of the four schools of jurisprudence.
2. The ruling of fasting the thirtieth of Sha`ban in case it is the first of Ramadan
It is haram to fast the thirtieth of Sha`ban (the day of doubt) out of fear it might be the first of Ramadan, or just in case.  This will be discussed in the section of haram fasting.
Thirdly: Astronomical Calculations
It is haram to rely on astronomical calculations to confirm the entering of Ramadan.  Relying on sighting the moon physically goes in line with moderate aims of the Law, since sighting the moon is comfortable and possible for all the Ummah, regardless of where they are. However, if calculations are relied upon, then this puts much of the Ummah in a disadvantageous position, since its majority does not know these calculations. The claim that the Ummah is no longer ignorant of this science is questionable, and even if we accept it for argument’s sake, the Law is for all times and places, and so does not change on whims. Consensus has been quoted over this by: al-Jassas, Ibn Rushd, al-Qurtubi, and Ibn Taymiyyah.  Ibn Baz said: “Whoever goes against this from the moderns, then he has been beaten by a consensus that came before him, and his opinion is rejected, as there is no discussion with anyone when it comes to the sunnah of the Prophet ﷺ, nor the consensus of the Salaf.” Majmu` Fatawa Ibn Baz (109/15).