How is the first day of the lunar month established? Can a person rely on astronomical calculations?
The basis for establishing the beginning of the lunar month is the appearance of the crescent moon on the horizon such that it can be sighted with the naked eye unless there are clouds or other obstacles. Thus, the beginning of the lunar month is proven by one of the following:
1. Knowledge from actually sighting the crescent with one’s own naked eye.
2. Knowledge [of the appearance of the crescent] received from multiple eyewitness accounts
3. Contentment attained from the spread of news among people.
4. Contentment attained from rational means.
5. The passing of thirty days since the start of the previous month.
6. The testimony of two just men who testify that they have sighted the crescent, provided their testimony is not contradicted by another testimony.