It is known that the lunar month – which is related to a group of rulings – is linked as far as its start and end are concerned with the movement of the moon, which completes its orbit around the earth in a period named ‘month’.
Astronomically, the month starts when the moon leaves muhaq (lit. complete omission, i.e. disappearance), which is the location in which the light of the moon is completely invisible to the onlookers on earth; when it starts leaving that location, its first appearance is known as ‘hilal’ (crescent). The month ends when the moon enters muhaq once again.
However, the Shariah does not depend on this to ascertain the start of the month, but it made it a condition that the crescent must developed enough to make it visible by onlookers to the horizon, after sunset, and when this takes place it is the beginning of the month.
To ascertain the start of the month according to the Shariah, there are several rulings:
581.The existence of the crescent may be ascertained by one of the following:
First:The person himself sees it.
Second:A large and diverse number of people giving witness in a way that leads to knowledge or satisfaction that the crescent exists, which is called ‘shaya‘’ (lit. something or a piece of news becoming common), so if neither knowledge nor satisfaction takes place, it is not allowed to depend on the witness, whatever the number of people.
Third:Witness given by two just men that they have seen the crescent; one witness is not sufficient, nor witness given by women, even if accompanying the men, unless satisfaction takes place from their witness.
Fourth: The elapse of thirty days from the beginning of the month.
Fifth:The ruling of the Islamic ruler/scholar (al-Hakim ash-Shar‘i), unless one has evidence of the incorrectness of the ruling or of the basis of the ruling.
Sixth:Every scientific method that leads to certainty or satisfaction that the moon has left the muhaq and that it exists on the horizon in a way that makes seeing it possible.
582.It is sufficient for ascertaining the crescent’s existence in a country to have it proven in another country which has a common part of the night with it, as long as sunset in the country where the crescent was declared present takes place before dawn in the country of the person.
583.When the crescent of the month of Ramadan is proven, fasting becomes obligatory, and when the crescent of the month of Shawwal is proven, breaking the fast becomes obligatory.
584.If the crescent of the month of Ramadan is not proven, it is not obligatory to fast during the day of doubt (i.e. the day which might be the last day of Shaaban, the eight lunar month, or the first day of Ramadan, the ninth lunar month), and it is not allowed to fast that day with the intention of asserting that it is a day of Ramadan. That said, it is recommended to fast on that day as a day of Shaaban; also, the person may fast on it with the intention of qadha’ (for a day he failed to fast in the past), then if it is proven to him that it was Ramadan, his fasting would count as Ramadan without any consequences. And if the crescent of Shawwal is not proven, the person must complete Ramadan's thirty days, then if during the day it is proven to him that it was Shawwal (i.e. the start of it), he must break the fast.