One quick refresher for nasif. With moon, there is nothing like "come up". Its either birth or visibility. Islamic events revolves arround the visibility (not the birth) of the cresent moon. After a new moon is born, certain amount of time (>16 hours, don't remember exactly) must pass by before it becomes visible with or without equipments. While the birth of the moon can easily be CALCULATED with a very HIGH degree of certainity in advance, the visibility can only be PREDICTED with SOME degree of certainity. Hope this will clear up some of your confusion.
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