1,344. The Visibility of the Moon in Lands to the West

Hilchos Kiddush HaChodesh 18:12

All of the preceding only applies when there’s a court that relies on the testimony of witnesses to sanctify the month. Nowadays, we rely exclusively on calculations of the moon’s average motion that are disseminated throughout Jewry, as has been explained.

Hilchos Kiddush HaChodesh 18:13

Works of astronomy and geometry explain that if the moon is visible in Israel, it will also be visible in all the lands that are west of Israel at the same latitude. If calculations determine that the moon will not be visible in Israel, the moon might still be visible in lands that are west of Israel at the same latitude. Therefore, if the moon is visible in a land to the west of Israel, it is not an indicator of whether or not it will also be visible in Israel.