Hilchos Kiddush HaChodesh 11:16
It has already been explained that the procedures described in these laws are only intended to determine when the moon will appear. Therefore, we need a point from which to begin calculating. The Rambam has selected Wednesday night (i.e., the evening leading into the fifth day of the week), 3 Nisan, of the 17th year in the 260th cycle, which is the Hebrew year 4938 from creation, year 1489 regarding the dating of legal documents and year 1109 from the destruction of the second Temple. This year will be called the starting point for this purpose.
Hilchos Kiddush HaChodesh 11:17
Since the appearance of the moon is only relevant in Israel as has been explained, all these calculations focus on Jerusalem and the places within a six- or seven-day journey. This is where the moon is regularly seen so that witnesses travel to testify about it in court. This is approximately 32 degrees north of the equator, with the outskirts ranging from 29 to 35 degrees. The longitude is approximately 24 degrees west from the population center (of the eastern hemisphere), with the outskirts ranging from 21 to 27 degrees.