Who's right about Ramadan?

[Khaybar]

After the End of the Holy month of Ramadan, there is the Fitr Eid or celebration, but the Sunnis said it was on Sunday, i don't have a problem there because trhe Sunnis began Ramadan earlier, but here in Lebanon Ayatollah FadlAllah, said it was on Monday, while Grand Ayatollah Ali Al-Sistani said it was tomorrow, the same as Khamenei, now Ayatollah FadlAllah beleives in coping Religion with modern science, and using telescopes and such to identifie the crescent, while Grand Ayatollah Ali Al-Sistani and Ayatollah Khamenei beleive in the use of the naked eye.

So who's right?

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تزول الجبال ولا تزل عض على ناجذك اعر الله جمجمتك تدفى في الارض قدمك ارم ببصرك اقصى القوم و غض بصرك و اعلم ان النصر من عند الله سبحانه
امير المؤمنين علي عليه السلام