As most of us know, the moon goes through several phases. The months of the Islamic Calendar are based on sighting of the new moon every month. In Astronomy, a new moon means “when the moon of the previous month disappears (i.e. the moon goes from a slightly visible crescent to a completely black sphere that is impossible to be seen).” Remember, new moon is dark and invisible. About 17-23 hours after the new moon, a thin crescent becomes visible on earth; this is visible new moon. As we were taught by Allah (Qur’an 2:189) and the Prophet Muhammad (peace be upon him), it is the sighting of this crescent that marks the start of a new month. Therefore, even though a moon may be born on one day, it may not been seen until the next day. Furthermore, if a new moon is born less than 15 hours before sunset on day 1; it will not been seen until sunset of day 2; and when it is finally seen it is 15+24=39 hours old and will look very thick. This does NOT mean that this is a second day moon. A first day crescent-moon can be very thin in some locations and it can also be thick in other locations because of time difference. As Muslims, we should always remember that our first priority is to follow the teachings of our beloved Prophet (peace be upon him) who told us to start the month when we see the crescent, not when the moon is “born.” If we do that, Insha-Allah we will start the month at the “right” time (the time that Allah intends for the month to start for a specific location).
(Courtesy of Moonsighting.com)