While trying to analyze the difference of opinion among the scholar on moon sightings, I came across the below Hadiths:
Abu Huraira reported Allah's Messenger (ﷺ) as saying:
Observe fast on sighting it (the new moon) and break (fast) on sighting it (the new moon), but if the sky is cloudy for you, then complete the number (of thirty).
The Prophet (ﷺ) said: Do not fast (for Ramadan) before the coming of the month until you sight the moon or complete the number (of thirty days); then fast until you sight the moon or complete the number (of thirty days).
Narrated Kuraib: That Umm al-Fadl, daughter of al-Harith, sent him to Mu'aqiyah in Syria. He said: I came to syria and performed her work. The moon of Ramadan appeared while I was in Syria. We sighted the moon on the night of Friday. When I came to Median towards the end of the month (of Ramadan), Ibn 'Abbas asked me about the moon. He said: When did you sight the moon ? I said: I sighted it on the night of Friday. He asked: Did you sight it yourself ? I said: Yes, and the people sighted it. They fasted and Mu'awiyah also fasted. He said: But we sighted it on the night of saturday. Since then we have been fasting until we complete thirty days or we sight it. Then I said: Are the sighting of the moon by Mu'awiyah and his fasts not sufficient for us? He replied: No. The Messenger of Allah (ﷺ) commanded us to do so.
Narrated Abdullah bin Umar:
Allah's Messenger (ﷺ) said, "The month (can be) 29 nights (i.e. days), and do not fast till you see the moon, and if the sky is overcast, then complete Sha'ban as thirty days."
My limited understanding of the subject leads me to believe that the Islamic month starts when the new moon is sighted. If it is not sighted due to clouds or some other reasons (volcanic eruptions, sand storms, smog etc), then the next night is considered to be first night of the new month.
But I'm confused when people say that even tough the moon isn't sighted it was born by astronomical calculations. Hence, Hilal is not more necessary. My knowledge tells me people of earlier times had great sophisticated knowledge when it comes to astronomy and surely at the time of the Prophet (SAW), people knew how to calculate lunar cycles, eclipses, movement of stars and constellations.
So is there something I am missing that the present day ulema are in disagreement over moon sighting and bring confusion among the masses atleast twice a year.