What is the criterion for sighting the new moon? The Grand Ayatollah Sistani’s answers

The Grand Ayatollah Sistani answered questions about sighting the new moon.

Question: Considering big differences in sighting the new moon:

1. What is the criterion for sighting?
2. Is it enough to use the existing calendars or it must be announced on behalf of Maraje’a (grand scholars)?
3. Isn’t sighting the moon in one country the criterion for other countries? How far is the horizon (skyline) of a country?
4. Must other towns in a country follow a town where the moon is sighted?

The Grand Ayatollah Sistani:

1. Seeing the moon with normal eyes without any equipment.
2. Calendar on its own is not credible.

3 & 4. Yes, like Karbala and Najaf if according to width of horizon are near each other.