Inquiries from his Eminence Ayatullah Sistani about crescent visibility and its proof
Therefore, based on such understanding, the information provided by astronomers about the birth of the crescent and its exit from the inter-lunar state does not suffice the start of a new lunar month even if it is based on certified mathematical calculations.
As for their information about the possibility of seeing the moon with the naked eye in some areas, either generally or given clear visibility conditions, as they say, it depends on two elements:
1) Specific astronomical calculations putting the crescent in those locations, given its age, degree above horizon, distance from the sun, and other factors influencing its visibility; and,
2) Astronomical experiments based on observing the crescent physically to confirm the attainment of the minimum requirements for visibility with the naked eye, given its age, degree above horizon, distance from the sun, etc.
The latter point is where the opinion of astronomers differ. For example, some indicate the possibility of sighting the crescent with the naked eye when it is 14 hours old, while others state the minimum time is 16 hours, and even others say 18 hours, and so on. Moreover, some claim visibility while the crescent is 4 degrees above the horizon at the time of sunset, while others say the minimum requirement for visibility is 5 degrees, and a group states it is 6 degrees, and others mention other degrees. Similar differences are found in other determining factors as well.
Based on that, a follower of the Islamic jurisprudence (mukallaf) cannot take the word of astronomers about the visibility of the crescent in so and so area, unless it is confirmed that the crescent can be clearly seen by the naked eye, based on narrations forbidding from reliance on opinion and speculation with regards to crescent sighting, such as the saying of Imam Al-Baqir (as), "When you see the crescent then fast, and when you see it then break your fast, and it is not about opinion or speculation but about visibility" (Tahtheeb Al-Ahkaam, vol. 4, p. 156).
However, if a follower of Islamic jurisprudence achieves confirmation or contentment (itmenaan), even if it is from experience or practice, that the crescent present on the current horizon, with so and so size (age) and degree (height above horizon) and all aspects influencing its visibility, is capable of being sighted by the naked eye, but was not seen due to clouds, fog, sand storm or other factors, then he/she must act according to the confirmation or contentment he/she achieved.
Question 2: It is said that his eminence (may Allah prolong his life) sometimes does not accept the testimony of witnesses about crescent sighting when it contradicts information provided by astronomers about the impossibility of seeing it, even though testimonies are sensory while astronomers' information are intuitive (i.e. based on speculation), so what is the reason behind that?
Answer 2: Two points to consider about calculations:
1) What is confirmed by mathematical calculations, and does not depend on personal speculation or interpretation, such as the time of the birth of the crescent, time of its exit from the interlunar state, the degree of its rise above the horizon, percent of illuminated circumference of the moon, and so on, where usually no conflict occurs between astronomers about such aspects, except maybe by miscalculation.
2) What is subject to personal interpretation and speculation and is based on experience and practice, such as some astronomers' statements about the impossibility of seeing the crescent unless it is 6 degrees above the horizon, or at an age of 22 hours, or at so and so distance from the sun, and so on, and in this point there is lots of disagreement and differing opinions (among astronomers).
Thus, if the testimonial of witnesses on crescent sighting is contradictory to the first type of information provided by astronomers (as mentioned in point 1 above), then one can confirm or be content (mutma'in) of the inaccuracy of the testimony. If they state (for example) sighting it while the crescent, according to meticulous calculations, is still in its inter-lunar state or that it sets before sunset, even if two or more testify sighting it, then (their testimony is dismissed).
If, however, testimonies are contradictory to the second type of the information provided by astronomers (as stated in point 2 above), then contentment might be achieved about the inaccuracy of the testimonials, looking at the proofs and evidence, or it may not be achieved. Therefore, if contentment about the inaccuracy of the testimonials is not achieved, and among the witnesses were two just people who their testimony fulfills the requirements of proof, then one must act according to such testimony and there is no room for doubt against it.
In summary, among the conditions for accepting the testimony of two just witnesses about crescent sighting is the lack of confirmation or contentment (itmenaan) that they are wrong, but if they can be proven wrong even by reliable information provided by an astronomer, for example the crescent is still in its inter-lunar state or that it is still very thin such that no such crescent was ever sighted before, then the testimonies do not carry any value (and are dismissed). Otherwise, one can take the testimony and ignore the astronomers' speculation.
Question 4: Why can we not follow the scientific advancements, I mean specialized instruments, with regards to sighting the moon on Eids and the beginning of the months, and we only rely on sighting with the naked eye?
Answer 4: The reason is because what is understood from the legislative narrations is that what matters in the start of the lunar month is the presence of the crescent in the horizon at the time of sunset at a degree of height and luminescence such that it is possible to observe by the common naked eye without presence of obstacles (i.e. clouds).
Question 6: If the crescent was sighted in one city, does that suffice for another city, or must both cities share the same horizon?
Answer 6: The same horizon must exist between both cities, meaning that visibility of the crescent in one city mandates its visibility in the other city as well if it were not for the presence of obstacles blocking sighting it such as clouds, a mountain and so on. This occurs when the crescent of the other city, according to careful astronomical calculations, has the same or even better (visibility) characteristics than the first city, looking at its size, height from horizon at the time of sunset and the degree away from the sun.
Question 7: What is the law about the crescent that is sighted in one place and not another?
Answer 7: Every place has its own ruling unless it is confirmed that both places share the same horizon.