Discussion on the unity of horizons for areas that share the night. 

The thing is that this Hadith would only apply if you can't see the moon/establish the start of the month. It never says to immediately rely on reports on neighbouring cities that share the night. 1st and main condition logically & Via hadith is to alwaya see the new month yourself/gain yaqeen for the sighting from Just witnesses etc. And if this can't be done, we then just complete 30 days of the mont then start the month accordingly.