question #1 Sahih al-Bukhari 1909, Book 30, Hadith 19 : What is the significance of sighting the crescent moon in determining the start and end of Ramadan?

The Prophet (ﷺ) or Abul-Qasim said, “Start fasting on seeing the crescent (of Ramadan), and give up
fasting on seeing the crescent (of Shawwal), and if the sky is overcast (and you cannot see it),
complete thirty days of Sha’ban.”