My understanding is that the reason for observing the 2nd Day of Yom Tov was that messengers were sent from the Sanhedrin to proclaim the day that Rosh Hodesh began, and they could reach only a certain distance within the same day.
What was that distance, originally - i.e. - during the time of the 1st Sanhedrin or as the Mishnah describes it. Has the distance where 2nd Day of Yom Tov applies changed since then, and is it different now?
Along with sources, if you can provide some landmarks (cities, villages, etc.) along with numerical distance, that would be useful.