This year (1960) the governments of Pakistan and Tunisia have decided to rely upon the opinion of astronomers for the confirmation of the new moon with a view of putting an end to confusion1 and the general inconvenience resulting from not knowing in advance the day of `Id, which at times comes as a surprise, and at other times is delayed despite all the preparations.
This decision of the two governments has become an issue of heated controversy in religious circles. The protagonists of the move observe that there is nothing in the religion that disapproves of reliance on the opinion of astronomers; rather it is supported by this verse of Surat al-Nahl:

    وَعَلامَاتٍ وَبِالنَّجْمِ هُمْ يَهْتَدُونَ

“...And way marks; and by the stars they are guided.” (16.16)

The antagonists state: The decision contradicts the above-mentioned prophetic traditon:

    صوموا لرؤيته وأفطروا لرؤيته

That, because the word ru'yah(sighting) implies sighting the moon with the eyes, which was common among the people during the time of the Prophet (s ). As to using a telescope or relying on astronomical calculations, they are inconsistent with the literal import of the tradition, they point out.

In fact, none of the sides has advanced sound reasons, because `guidance by the stars' implies determination of land and sea routes with the help of the stars, and not determination of days of months and new moons. As to the tradition, it does not contradict sound scientific knowledge, because `seeing' is a means for acquiring knowledge and not an end in itself, as is the case with any means that helps confirm facts.

However, in my opinion, the judgment of astronomers do not lead to certain knowledge, nor do they remove all doubts as removed by vision, because their judgments are based on probability not on certainty. This is evident from their divergent judgments about the night of the new moon as well as the time of its occurrence and the period that it remains (above the horizon).

If a time comes when the astronomers attain accurate and sufficient knowledge, so that there is consensus among them and they repeatedly prove to be right to the extent that their forecasts become a certainty like the days of the week, then it will be possible to rely upon them. Rather, then it will be obligatory to follow their judgments and to reject everything that goes against them.2

Notes
1. In 1939 the `Id al- Adha was observed on Monday in Egypt, on Tuesday in Saudi Arabia, and on Wednesday in Bombay.
2. Refer to the discussion on this issue in the first volume of our book Fiqh al-'Imam Jafar al-Sadiq (`a), the section on the proof of the new moon at the end of section on fasting bab al-sawm.

Source: Fasting According to the Five Schools of Islamic Law, Muhammad Jawad Mughniyyah.