The New Moon cannot be seen usually before sunset, since the light of the sun prevents its sighting, and if it is seen after sunset it proves that the next day is the first day of the month.
The completion of the religion on the day of Al-Ghadeer was the appointment of the Imam to succeed the Prophet (peace be upon him and his holy progeny) and represent him in propagating the Sharia and leading the nation, and then the rest of the Imams were appointed as well. The religion was completed when the Muslims were directed to refer to the Imams after the prophet (peace be upon them all). This provided the Islamic nation with more than two centuries of infallible leaderships, during which the Imams (peace be upon them) did their best in informing all the aspects of the religion especially the practical laws of the Sharia. The companions of the Imams propagated their teachings, and compiled the narrations (ahadeeth) in books. Although some of these books were destroyed, these traditions and narrations have survived and still act as a main source of Islamic research. Scholars have continuously strived to pass on these teachings from generation to generation for the sake of protecting the religion and its laws. As a result, the jurist can give their edicts in any practical issue, in such a way that the individual is not left unanswered.
This is not necessary.
Our edicts are based on the teachings of the Imams of the Ahlulbait (peace be upon them) through the narrations that reached us in reliable ways, which all Muslims are required to act upon. The Prophet (peace be upon him and his holy progeny) ordered the nation of Islam to refer to the Ahlulbait and follow them in the famous well-known narration that is reported by numerous ways in the books of Shias and Sunnis: "I leave among you the two weighty things: the Book of Allah and my progeny; as long as you follow both of them, you will never be misguided." Acting upon this narration, as well as other similar narrations, to follow the AhlulBait is not obligatory on the Shias only, but it is an obligation on all Muslims.