Significance of "AL URJOONAL QADEEM"
Al Urjoonal Qadeem like moon-phase is the only phase mentioned & vividly described by Almighty Allah in Noble Qur'an (verse 36:39). This is the last visible phase for a month. If one observes moon after Last Quarter, at the time of Fajr, one can see the big crescent becoming thinner & coming closer to Eastern horizon by each passing day and on pre-ultimate day of the month it will appear as Al Urjoonal Qadeem, so thin & so closer to the horizon that one is assured of not seeing the waning moon with naked eyes on the following day which is the Day of Conjunction or the last day of the month. (Refer: Explanation of verse 71:16 in Tafsir Ibn Katheer). As all will agree, once the last day is identified then it is very easy to start the new month on the correct day.
The advantage of observing the waning phases, instead of first waxing crescent , is that the moon appears to descend each day almost 12 °, similar to someone descending a staircase, one step a day. Hence even if one misses the sight of the moon for 1 or 2 days one can easily ascertain, establish and pinpoint the day on which Al Urjoonal Qadeem will appear. After almost 48 hours of its appearance, the new month starts all over the world.
In other words this method of sighting Al Urjoonal Qadeem is as simple and convenient as waiting for a train at the platform well in advance whereas starting the new month after seeing the setting moon on West is similar to reaching Railway tation after the day's train has left compelling one to board only on next day's train !!
Hence observing & identifying Al Urjoonal Qadeem is significant as it heralds or ushers in the new month; giving well in advance the information of correct starting day of new month.
More over, as per authentic hadeeth our beloved Prophet (PBUH) used to observe all moon-phases throughout the year especially those of Sha'baan. So, it is Sunnah to observe all the moon-phases, including Al Urjoonal Qadeem.