Lunar calendar

The moon in a full phase
Al-Hilal: The first visible moon on Earth

A global Islamic calendar can be published by precalculating the presence of the Al-Hilal (the first visible crescent moon on earth).
Ramadan tradition in Muslim world
Ramadan and the Lunar Calendar

We all know that Ramadan is the 9th month of the Islamic or Hijra calendar, but how and when did this calendar come to be used by the Muslims?