Rü'yet-i Hilal Meselesi

The Visibility of the New "Moon" (Ru'Yat al-Hilal)
In accordance with the act of praying and some judgments of the actions it is very important to define the beginning of the lunar months correctly in Muslim's daily life. Nevertheless one of the current problems of Muslim is their different definition about the beginning of Ramadan. This differentiation was normal and reasonable for the past periods. Today, however, with regard to astronomical developments the declaration of the beginning of Ramadan within three or four different days can not be justified and not be seen reasonable. By the time of the holy Prophet the definition of the beginning of the lunar month was based on an easy and understandable method for practicing of the people. This method based on the observation of the new moon with eye in order to define the beginning of the lunar month i. e. Ramadan and under the condition if the new moon is invisible it must be completed thirty days. So this method of the definition had been used for ages.
The calculation of the beginning of the lunar month is more reliable today than the observation of the new moon with eye. Now it must be regarded that the new moon can be specified by calculation.

https://www.marife.org/marife/article/view/67