question #1 Sahih Muslim 1081d, Book 13, Hadith 23 : What is the significance of sighting the new moon in determining the beginning and end of Ramadan?

Observe fast when you see it (the new moon) and break fast when you see it (the new moon of Shawwal), but when (the actual position of the month is) concealed from you (on account of cloudy sky), then count thirty days.